<section data-role="outer" label="Powered by 135editor.com" style="font-size: 16px;"><p style="text-align: center;"></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="background-color: rgb(255, 255, 255);color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;"></span><br></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="background-color: rgb(255, 255, 255);color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;">对于SQl, 很多人学不会的原因是从一开始就没明白，学这东西能干啥，学会了能有什么用。甚至有些人不知道‘SQL’应该怎么读，以至于一开始兴致勃勃，但是学到一半放弃了。</span><br></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;word-wrap: break-word !important;"><br></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">注意：‘sql’真的不能读成‘烧烤’、‘惜烤’、’撒扣’、’斯购‘，另外，’MYSQL’真的不能读成’卖烧烤’，’卖西裤‘，有学员问：那我卖什么，我回答说：你去死。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;word-wrap: break-word !important;"><br></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">如果你真的、实在是、绝对的看不懂英文的音标，那就暂且读’色儿扣‘吧，这是针对英语文盲特别制作的读法。（注意我在这里并没有瞧不起你的意思）</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;word-wrap: break-word !important;"><br></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">所以，坚持学会的根本，不在于这个知识是否有多难，而在于它带给你的意义有多大。如果做事情没有意义，那么你就没有学习的动力。在数据分析的招聘中，SQL是必考的能力之一。所以学习SQL的意义，就在于你能否通过数据分析的面试，如果不能通过面试，你将穷困潦倒，颠沛流离，别人娶妻生子，儿孙绕膝，而你孤苦伶仃，满目凄凉，垂垂老矣....</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;word-wrap: break-word !important;"><br></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">想要入门SQL，关键在于有没有一个好的领路人带你入门，所以ladys and乡亲们，放下手头所有的农活，开始喜欢我吧，因为接下来，我将带你轻松搞定SQL的各个语法。记住我最大的特点就是，通俗易懂的解释一个事情。</span></p><p><br></p><p style="margin: 15px 8px;max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;white-space: normal;widows: 1;text-align: center;background-color: rgb(255, 255, 255);line-height: 1.75em;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="max-width: 100%;font-size: 50px;color: rgb(255, 120, 0);box-sizing: border-box !important;word-wrap: break-word !important;">01.</span></p><p style="margin: 15px 8px;max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;white-space: normal;widows: 1;text-align: center;background-color: rgb(255, 255, 255);line-height: 1.75em;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="max-width: 100%;color: rgb(0, 0, 0);font-size: 16px;box-sizing: border-box !important;word-wrap: break-word !important;"><strong style="max-width: 100%;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="max-width: 100%;letter-spacing: 0.5px;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;font-weight: 700;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">理论篇</span></span></strong></span></p><p style="margin: 15px 8px;max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;white-space: normal;widows: 1;text-align: center;background-color: rgb(255, 255, 255);line-height: 1.75em;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="max-width: 100%;color: rgb(0, 0, 0);font-size: 16px;box-sizing: border-box !important;word-wrap: break-word !important;"><strong style="max-width: 100%;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="max-width: 100%;letter-spacing: 0.5px;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;font-weight: 700;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">理解重要的概念</span></span></strong></span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;line-height: 22px;box-sizing: border-box;word-wrap: break-word !important;"><br></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(255, 76, 0);"><strong><span style="font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【数据】</span></strong></span><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">数字，文字，图形，图像，声音这些都叫数据。</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.4081196581196581" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_jpg/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxiaHZpvWLTskEM6ZscdwAavxrzhlcaiaiaUqTtSAaneKrRib7giaDSAeBRibQ/640?wx_fmt=jpeg" data-type="jpeg" data-w="468" style="width: 468px !important; height: 191px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920135519962-983457629.png" crossorigin="anonymous"></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><span style="background-color: rgb(255, 255, 255);color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;text-align: justify;"><br></span></p><p style="text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><strong><span style="background-color: rgb(255, 255, 255);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;text-align: justify;color: rgb(255, 76, 0);">【数据库】</span></strong><span style="background-color: rgb(255, 255, 255);color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;text-align: justify;"> 理解数据库的一种最简单的方法是 将它想象成一个电子化的“文件柜”，</span></p><p style="text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><span style="background-color: rgb(255, 255, 255);color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;text-align: justify;"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">这个“文件柜”是一个存放数据的物理位置，不管数据是什么形式！就像法海手里的金铂，我管你是什么妖，统统都得收进来!（电视剧里的那个金铂看起来像一个电饭锅的内胆，有没有很像）。也有人说数据库存储数据，就像冰箱是存储食物一样，，什么肉啊，米啊，菜啊，都能存放，对此我也是十分十分的同意！！</span></p><p class=""><br></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.5925" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_jpg/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxEUF7SfwibkibsDlkMq9xrlb7cCtFYZRQjJMibZN4Wficoial18JnkYloPmQ/640?wx_fmt=jpeg" data-type="jpeg" data-w="400" style="width: 400px !important; height: 237px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920135558736-540616755.png" crossorigin="anonymous"></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">有了数据库后，所有终端设备（手机，PAD, PC）都可以直接在这个系统上查找数据和修改数据。有人问我，老师，我是安卓手机，能连接数据库吗，当然能了，不许瞧不起安卓系统！！</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.42280285035629456" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxQgkKG1ushVCrFherlic8nRTzaSnOOt2VSLUodmWNibsOVSrARD3RDlWQ/640?wx_fmt=png" data-type="png" data-w="421" style="width: 421px !important; height: 178px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920135656203-984228566.png" crossorigin="anonymous"></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><span style="background-color: rgb(255, 255, 255);color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;text-align: justify;"><br></span></p><p style="text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><strong><span style="background-color: rgb(255, 255, 255);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;text-align: justify;color: rgb(255, 76, 0);">【数据表】</span></strong><span style="background-color: rgb(255, 255, 255);color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;text-align: justify;"> 表是由行和列组成的一张二维表，这是关系数据库中最基本的数据模型</span></p><p class=""><br></p><p style="text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.48282097649186256" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxh0Pje8gf1HGXl1MqV5GqMib5PppDveGQKaPu4hHKicWLfibiaXcZCdqyGA/640?wx_fmt=png" data-type="png" data-w="553" style="width: 553px !important; height: 267px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920135718602-688250737.png" crossorigin="anonymous"><strong><span style="background-color: rgb(255, 255, 255);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;text-align: justify;color: rgb(255, 76, 0);">【主键】</span></strong><span style="background-color: rgb(255, 255, 255);color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;text-align: justify;">用来表示数据的唯一性，是数据库中非常重要的概念。在生活中我们也会使用ID来标识数据的唯一性，例如：国家用身份证来唯一标识一个人，即使你有一个双胞胎兄弟，但只要你们的身份证号不一样，你们就是两个人。再比如银行卡的卡号就是主键，如果两个人卡号重复了，里面有100万，钱算谁的呢？这事就闹大了。</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.8349514563106796" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxby2iaZtAfRPgoyMbE6sNkX5ObZfsicWDG0OsLLb2Z6tGtQSsic5oq8GxQ/640?wx_fmt=png" data-type="png" data-w="309" style="width: 309px !important; height: 258px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920135741015-1520236742.png" crossorigin="anonymous"></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(255, 76, 0);"><strong><span style="font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【外键】</span></strong></span><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"> 关联到其他表主键的字段被称为“ 外键 ”，如果A表的一个字段来源于B表，那么这个字段就是A表的外键。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">例如：成绩表中的字段学号 sno 就是外键， 因为这个字段的值来源于学生表的主键sno。</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.5655021834061136" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxw4vbbzTuabG94953h62SSncZgibD2dFqCMfG3oWaQ7nOiazuDNaYPokQ/640?wx_fmt=png" data-type="png" data-w="458" style="width: 458px !important; height: 259px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920135828696-1989364955.png" crossorigin="anonymous"></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(255, 76, 0);"><strong><span style="font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【表关联】</span></strong></span><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">什么是联系呢？你是你爸爸的儿子，你是你的儿子的爸爸。这就是生活中的联系。其实，数据之间也是有联系的。数据库里各个表之间如何建立联系呢？</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">将多张表通过主键+外键关联起来的方式，叫作表关联。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">例如学生选课涉及的四张表，通过主键和外键关联在一起，就构成了一个简易的学生选课数据库系统。</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.6824196597353497" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_jpg/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxOOhMq9RscdFYTkr8sbf4nPY7SkkfY3AwLPQ01N7BUwM97MCgSyvIyQ/640?wx_fmt=jpeg" data-type="jpeg" data-w="529" style="width: 529px !important; height: 361px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920135855397-190525347.png" crossorigin="anonymous"></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">表关联是关系数据库中非常核心的理念 ，它可以表达非常复杂的数据关</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="background-color: rgb(255, 255, 255);color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(255, 76, 0);"><strong><span style="background-color: rgb(255, 255, 255);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;">【概念模型】</span></strong></span><span style="background-color: rgb(255, 255, 255);color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;">表示方法：E—R方法(也叫实体联系法)</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.39544513457556935" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxun1crDJ77d5OuLLanLgmaZP7XNYW6Q1WUb9A3CST3IibrQ6UoLbz4qg/640?wx_fmt=png" data-type="png" data-w="483" style="width: 483px !important; height: 191px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920135930022-1392892282.png" crossorigin="anonymous"></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【一个E-R模型案例】:</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.8491879350348028" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_jpg/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhx0oiaHJkgKiazUoEFdtfRKX3SicSsUgdohppB9B8xZTib9qNLrqoUy9p8kA/640?wx_fmt=jpeg" data-type="jpeg" data-w="431" style="width: 431px !important; height: 366px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920135951329-489217512.png" crossorigin="anonymous"></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(255, 76, 0);"><strong><span style="font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【数据模型】</span></strong></span><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">（主要指关系模型）</span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">SQL语句的基础是关系数据库，关系数据库的基础是关系模型，关系模型可以由E-R图得到。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">在关系数据库中，一张表就是一个关系，所以表格的一些术语和关系的一些术语能表达同样的意思，具体来说，有下面几点不同：</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.5397111913357401" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_jpg/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxHuiaIGxU9aJFxRX6tYYR08yJ9COdU8GmrAgecBd5lW3yvLept4icwkuQ/640?wx_fmt=jpeg" data-type="jpeg" data-w="554" style="width: 554px !important; height: 299px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920140013767-1034457158.png" crossorigin="anonymous"></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">上面讲的都是数据库方面的基本理论。理论有了，当然就得有对应的软件实现才能用起来，不然再强大的理论都是一坨屎。这就好比，如果只有设计草图是无法盖起楼房的，得有具体的施工团队才能盖起楼房。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">上面讲的数据库原理就是“设计草图”，那么对应的“施工团队”是谁呢？</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">实现数据库原理的“施工团队”就是常用的数据库软件，例如MySQL、Oracle、SQLServer等都是实现上面理论的数据库。各个数据库软件的使用差别大同小异，但是因为MySQL是开源的，免费的，所以成为各大公司使用的主流，面试也主要以’卖烧烤‘为主。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="margin: 15px 8px;max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;white-space: normal;widows: 1;text-align: center;background-color: rgb(255, 255, 255);line-height: 1.75em;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="max-width: 100%;font-size: 50px;color: rgb(255, 120, 0);box-sizing: border-box !important;word-wrap: break-word !important;">02.</span></p><p style="margin: 15px 8px;max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;white-space: normal;widows: 1;text-align: center;background-color: rgb(255, 255, 255);line-height: 1.75em;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="color:#000000;"><span style="letter-spacing: 0.5px;"><strong>SQL基础篇</strong></span></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">我经历过很多次数据分析师面试，经常会遇到这样一道经典的SQL笔试题。注意：这可是华为的面试题，上点心吧。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【华为经典面试题】</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">A（用户购买记录）、B（商品对应ID）、C（ID对应价格）三个表，问本月内购买商品金额最多的前三名用户分别是谁？</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【分析】</span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">A表只有用户和商品名字的对应关系，需要关联到C表求得价格，但A表和C表没有公共字段，这时发现A表和B表有公共字段【商品名字】，而B表和C表有公共字段【商品ID】，于是先用A表Join B表，再Join C表得出一张多个字段的关联表，用Group by求出用户购买商品的总金额，再用Order by对用户购买商品的总金额进行降序排列，最后用Limit截取总金额前三位的用户。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">这道题看起来很难，但考到了都是一些SQL的基础知识，包括Select、Groupby、Order by、Join、Limit。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">所以，作为数据分析狮（有人喜欢把自己叫作分析狗，真是太不尊重自己了，我听了非常气愤），如果你以后的发展方向不是数据工程师的话，SQL只需要掌握基础语句就可以糊弄面试了，具体包括：</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">查询数据（select语句）</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">过滤数据（where语句）</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">汇总数据（group by 语句）</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">排序数据（order by 语句）</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">时间函数</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">1 查询数据（select语句）</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【语法】select 列名称 from表名称</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【案例】select name from world; (选取world表中‘name’这一列)</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">select * from world(选取world表中所有列)2 过滤数据（where语句）</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【语法】select xxxx from xxx Where 条件表达式</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【案例】select * from country where IndepYear >1975</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.40828402366863903" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxx9XQ7Dd4VSaulYqGn9nWBRRGj4TvjHhILpZgwEdttWKd2Nyaaia8eQg/640?wx_fmt=png" data-type="png" data-w="507" style="width: 507px !important; height: 207px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920140137817-823140211.png" crossorigin="anonymous"></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">3 使用distinct关键词去重：</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【语法】select distinct 字段名 from 表名</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【案例】select distinct class from student</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.37636761487964987" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxcbUQ0jCDvKl1s6xZ3jxCKJia71UxxTsErbLWI1UPlUHGWNeMtgdLbrA/640?wx_fmt=png" data-type="png" data-w="457" style="width: 457px !important; height: 172px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920140202311-1725827803.png" crossorigin="anonymous"></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="background-color: rgb(255, 255, 255);color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;">汇总数据（ Group by 语句）</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【定义】什么是group by 子句，“Group By”从字面意义上理解就是根据“By”指定的规则对数据进行分组. 那么，GROUPBY 是如何分组的呢？</span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【案例】</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.6057866184448463" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_jpg/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxRT63Syic4cR58ibGmEnpLTKq5MibviaF5pYQxFlVy8GibJerxrE1JMoiaYZA/640?wx_fmt=jpeg" data-type="jpeg" data-w="553" style="width: 553px !important; height: 335px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920140225939-1210662524.png" crossorigin="anonymous"></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">所以group by也叫分组函数。5 排序语句（order by）</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">Order by 指的是根据指定的列对结果进行排序，默认升序；如果希望降序，需要加上DESC关键字。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【语法】selectxxx from xxx</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">where xxx</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;text-align: left;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">order by xxx desc</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【案例】筛选人口总数大于10万的国家的信息，并且按照人口总数进行降序排序。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">select * from countrywhere population > 100000</span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">order by population desc (先筛选符合条件的行，后排序)</span></p><p><br></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">使用Orderby 的几种情况：(1)order by 作用于单个字段</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.2513562386980108" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxhL9G5MQqicaH7xFckNT9Derf76ZO3UOoTUicH4Yhvtygo08dFEQhqC8A/640?wx_fmt=png" data-type="png" data-w="553" style="width: 553px !important; height: 139px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920140248359-1229697715.png" crossorigin="anonymous"></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">(2)加desc关键字实现降序</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.30018083182640143" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxX9Qm0OzslGARuyndrdhd6q1oFJ1cibvESZ2jrLG2oYg6SN9cia0S6CWg/640?wx_fmt=png" data-type="png" data-w="553" style="width: 553px !important; height: 166px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920140314733-420679069.png" crossorigin="anonymous"></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">(3)order by 作用于双字段</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.20938628158844766" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxCRGUQVq3bkQicNHlOrIEs6mSwRDfaz8CuJLPEJV4GqbshSGHpxTapug/640?wx_fmt=png" data-type="png" data-w="554" style="width: 554px !important; height: 116px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920140335920-1596066063.png" crossorigin="anonymous"></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">先对年龄进行升序排序，然后针对相同年龄的，再根据成绩升序排序</span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">6时间函数</span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">关于阿里，京东，美团，快手等大部分互联网公司，大部分数据需求都是和时间相关的，所以熟练掌握时间函数，非常的重要！！有人告诉我他不想工作，我奉劝你照照镜子，看看镜子中的自己，已经长得这么丑了，难道还要没钱吗？</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">真是no zuo no die，我不会骂你的，我很有修养</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="margin: 15px 8px;max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;white-space: normal;widows: 1;text-align: center;background-color: rgb(255, 255, 255);line-height: 1.75em;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="max-width: 100%;font-size: 50px;color: rgb(255, 120, 0);box-sizing: border-box !important;word-wrap: break-word !important;">03.</span></p><p style="margin: 15px 8px;max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;white-space: normal;widows: 1;text-align: center;background-color: rgb(255, 255, 255);line-height: 1.75em;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="max-width: 100%;color: rgb(0, 0, 0);font-size: 16px;box-sizing: border-box !important;word-wrap: break-word !important;"><strong style="max-width: 100%;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="max-width: 100%;letter-spacing: 0.5px;box-sizing: border-box !important;word-wrap: break-word !important;">SQL进阶篇</span></strong></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;word-wrap: break-word !important;"><br></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">恭喜你，你已经看到进阶篇了，现在你已经get到一个了不起的技能，妈妈再也不用担心你没法装X了。下面这些资料是我们内部的一些分享资料，有人问我什么内部，你在哪个公司上班，拜托大家不要像我二姑妈似的这么八卦，要学会看重点！！进阶部分需要大家了解mysql的执行原理，join的用法，SQL的执行顺序等等，因为这是我们写复杂SQL的基础。你不会走，就想飞吗，真的没这么容易！！</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">（1）mysql中join的用法</span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">之前网络上流行用韦恩图来表示各种连接查询：</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.8103448275862069" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_jpg/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxEVtRwzsXXoksTAWxsplqI0roodl6k6sgXte4IzjmTdGiaQhBFtBZAYw/640?wx_fmt=jpeg" data-type="jpeg" data-w="406" style="width: 406px !important; height: 329px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920140410499-1948057571.png" crossorigin="anonymous"></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">但我觉得用下面这种方式表达连接查询，更容易理解：</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.3447653429602888" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxTGN3DyJdGnYz1x8dda2XD3bKYXKXOFCByRv6Af79SaAhG0MiaK7DGZw/640?wx_fmt=png" data-type="png" data-w="554" style="width: 554px !important; height: 191px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920141843407-447030745.png" crossorigin="anonymous"></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.37725631768953066" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxncWC06sy9a7rh7Jryg0jZ7CDFIv270CwKN7WXMqg8uNib8BiauxrjW8A/640?wx_fmt=png" data-type="png" data-w="554" style="width: 554px !important; height: 209px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920141912308-759420408.png" crossorigin="anonymous"></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.33273056057866185" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxcuthJWu3FVN4gLXlJ1yApVy6L4zmPBaubKia0Kckp5WPeX2VZG1rOfQ/640?wx_fmt=png" data-type="png" data-w="553" style="width: 553px !important; height: 184px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920141940213-795484002.png" crossorigin="anonymous"></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.3833634719710669" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhx490XvL6BLhcb0QEdVCVdW3Qb2fzP8TRQY5UhgY0dYV0ibQicL6Ebo8Aw/640?wx_fmt=png" data-type="png" data-w="553" style="width: 553px !important; height: 212px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920142008101-121881668.png" crossorigin="anonymous"></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.5108303249097473" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxRIyrF3dnVDaOEcmgZ7IXcPjseib6IFHnnQh6icDkXw1H0fU1CKeEqoFQ/640?wx_fmt=png" data-type="png" data-w="554" style="width: 554px !important; height: 283px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920142027918-1725561502.png" crossorigin="anonymous"></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">有没有觉得连接查询竟然如此简单，还不是多亏了我这几张图吗。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">（2）Mysql的执行顺序：</span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">当sql逻辑复杂后，sql的执行顺序就会非常重要，SQL执行顺序和书写顺序又不一样，所以慢慢扣吧，这也是进阶的基础呢。</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.4423076923076923" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_jpg/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxZGltzSWt7g5lYyPc0MFuPx7ajy0t54lRNhibu3AAGzjOkic5AeM6IicYg/640?wx_fmt=jpeg" data-type="jpeg" data-w="520" style="width: 520px !important; height: 230px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920142047984-1137671831.png" crossorigin="anonymous"></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="margin: 15px 8px;max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;white-space: normal;widows: 1;text-align: center;background-color: rgb(255, 255, 255);line-height: 1.75em;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="max-width: 100%;font-size: 50px;color: rgb(255, 120, 0);box-sizing: border-box !important;word-wrap: break-word !important;">04.</span></p><p style="margin: 15px 8px;max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;white-space: normal;widows: 1;text-align: center;background-color: rgb(255, 255, 255);line-height: 1.75em;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="max-width: 100%;color: rgb(0, 0, 0);font-size: 16px;box-sizing: border-box !important;word-wrap: break-word !important;"><strong style="max-width: 100%;box-sizing: border-box !important;word-wrap: break-word !important;"><span style="max-width: 100%;letter-spacing: 0.5px;box-sizing: border-box !important;word-wrap: break-word !important;">SQL复杂查询</span></strong></span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span><br></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">恭喜你，你都看到复杂查询了，我觉得你很有潜力！!面对大量数据我们时常需要对他们进行汇总，分组，归类，这时候就需要用到一些复杂的查询方法。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">1聚合函数：【分类】聚合函数有哪些？</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.8625" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_jpg/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxEg25vLxArP0ib0epF3XvE1xicDcicPkmTcqy7QsyX0Ul5gXdSKWcdBN2w/640?wx_fmt=jpeg" data-type="jpeg" data-w="400" style="width: 400px !important; height: 345px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920142110639-1441404573.png" crossorigin="anonymous"></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">2分组聚合</span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【定义】指的是先使用group by 分组后，再使用聚合函数对每组数据聚合</span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">分组语句和聚合函数常常一起使用</span></p><p class=""><br></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.4989059080962801" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxxFkdy6vlgVPhVNevYmfBAfvqbTZt1uT0HXic8wddboJSYLtOrEkBy5A/640?wx_fmt=png" data-type="png" data-w="457" style="width: 457px !important; height: 228px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920142133908-1330280120.png" crossorigin="anonymous"></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【案例】：</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.5623869801084991" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_jpg/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxbZBjgFPRtGQ9Qt5KlFs0JcXNgYZYjskMDX4C2v7E5lx7eLNxBicfFKg/640?wx_fmt=jpeg" data-type="jpeg" data-w="553" style="width: 553px !important; height: 311px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920142157019-1429096789.png" crossorigin="anonymous"></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">3 使用having子句</span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【定义】筛选符合条件的组，常常和group by 一起使用。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【示意图】</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.628158844765343" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxnicSMMEHibOpljeckR1l5iaaicfo03hGJFiaJvXtFfhKYMicUeC4dmeviaB4g/640?wx_fmt=png" data-type="png" data-w="554" style="width: 554px !important; height: 348px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920142219012-1108706528.png" crossorigin="anonymous"></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">4嵌套查询：</span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">（1）【定义】什么是嵌套查询？</span></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">如果把一个查询块嵌套在另一个查询块的WHERE子句或HAVING短语的条件中，我们就称为嵌套查询。其中外层查询称为主查询，内层查询称为子查询。</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.5350877192982456" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhx1VCxnrzda6HgdcH9BR6D5FRtZciayMp5c457ibns0SiapPxew4usBVfgw/640?wx_fmt=png" data-type="png" data-w="456" style="width: 456px !important; height: 244px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920142240120-1203431246.png" crossorigin="anonymous"></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><strong><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">（2）【执行顺序】先执行内层查询，再执行外层查询：</span></strong></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.5268817204301075" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_png/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxAMBdicAichTdmX1V7XMb0K9ibibvCliaydSnsENq5UDa6UibCsW7EpJzqTAw/640?wx_fmt=png" data-type="png" data-w="465" style="width: 465px !important; height: 245px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920142302444-2031746129.png" crossorigin="anonymous"></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">（3）嵌套查询用来解决哪些问题？</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.7511737089201878" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_jpg/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxfUXhE5WaQKKFKWXv1acWkTPovc0icQZI1FRBp6Liav956iaZriaHIcK0tw/640?wx_fmt=jpeg" data-type="jpeg" data-w="426" style="width: 426px !important; height: 320px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920142331452-1287107110.png" crossorigin="anonymous"></p><p style="font-weight: bold;color: rgb(136, 136, 136);font-size: 14px;box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">【Sql 语句】</span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-ratio="0.6841155234657039" data-s="300,640" data-src="https://mmbiz.qpic.cn/mmbiz_jpg/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxzgCfzrvUBWpA8YGRuyfc7kBYbPaXO460UW02xORRfC4BAU6lNlQn0A/640?wx_fmt=jpeg" data-type="jpeg" data-w="554" style="width: 554px !important; height: 379px !important;" _width="auto" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920142401532-487349221.png" crossorigin="anonymous"></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">掌握了上面这些语法，基本上你可以糊弄各种形式的面试了，最常见的考SQL的方法就是，给你一个虚拟的数据库表结构，然后让你按给出的条件查询出数据，并用手写的形式写出在纸上。所以面试前把常用的SQL语句记清楚就OK了。</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">怎么，觉得自己牛逼的不行，再练点更难的题吧</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">推荐一个很方便的实践SQL的网站：数据库SQL实战-牛客网</span></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);"><br></span></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><img class="img_loading" data-croporisrc="https://mmbiz.qpic.cn/mmbiz_jpg/cx8SumXqBLrOHy7rlrYcSAktuEjqCjhxdPDjerVVKDa9dBGdO4UFPeDKceyYqYGjoIeAbuZ5PAQVAYfhx2rNFw/640?wx_fmt=jpeg" data-cropx1="0" data-cropx2="553" data-cropy1="69.63703703703705" data-cropy2="393.24444444444447" data-ratio="0.5858951175406871" data-s="300,640" data-src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920142426910-805238104.png" data-type="jpeg" data-w="553" style="width: 540px !important; height: 316.383px !important;" _width="540px" src="https://img2018.cnblogs.com/blog/1112483/201909/1112483-20190920142426910-805238104.png" crossorigin="anonymous"></p><p style="text-align: center;margin-left: 8px;margin-right: 8px;line-height: 1.75em;"><br></p><p style="max-width: 100%;font-size: 14px;color: rgb(136, 136, 136);box-sizing: border-box;margin-left: 8px;margin-right: 8px;line-height: 1.75em;word-wrap: break-word !important;"><span style="color: rgb(62, 62, 62);font-family: Arial, Helvetica, sans-serif;font-size: 15px;letter-spacing: 1.5px;widows: 1;background-color: rgb(255, 255, 255);">实际上还有很多类似这样的网站，是用来强化SQL练习的，但因为他们并没有给我广告费，我为什么要给他们做广告呢？</span></p></section>
原文地址：https://mp.weixin.qq.com/s/t8JCJSP__qh11U2zl8hCeQ </div></div>
