数据库学习 select
前言
数据库是必备知识，更是面试时必问，要会最基本的增、删、改、查，推荐一个刷SQL题的网站http://xuesql.cn/lesson/filtering_sorting_query_results
写此文是记录一下，SQL的知识点，以便今后查漏补缺
数据库查询：（Select)
基本查询语句：
Select (列名）from （表名）
SELECT * FROM mytable（表名）下面的具体语法规则，可以用来筛选数字属性列（包括 整数，浮点数） :
| Operator（关键字） | Condition（意思） | SQL Example(例子） |
| =, !=, < <=, >, >= | Standard numerical operators 基础的 大于，等于等比较 | col_name != 4 |
| BETWEEN … AND … | Number is within range of two values (inclusive) 在两个数之间 | col_name BETWEEN 1.5 AND 10.5 |
| NOT BETWEEN … AND … | Number is not within range of two values (inclusive) 不在两个数之间 | col_name NOT BETWEEN 1 AND 10 |
| IN (…) | Number exists in a list 在一个列表 | col_name IN (2, 4, 6) |
| NOT IN (…) | Number does not exist in a list 不在一个列表 | col_name NOT IN (1, 3, 5) |
模糊匹配：
| Operator（操作符） | Condition（解释） | Example（例子） |
| = | Case sensitive exact string comparison (notice the single equals)完全等于 | col_name = "abc" |
| != or <> | Case sensitive exact string inequality comparison 不等于 | col_name != "abcd" |
| LIKE | Case insensitive exact string comparison 没有用通配符等价于 = | col_name LIKE "ABC" |
| NOT LIKE | Case insensitive exact string inequality comparison 没有用通配符等价于 != | col_name NOT LIKE "ABCD" |
| % | Used anywhere in a string to match a sequence of zero or more characters (only with LIKE or NOT LIKE) 通配符，代表匹配0个以上的字符 | col_name LIKE "%AT%" (matches "AT", "ATTIC", "CAT" or even "BATS") "%AT%" 代表AT 前后可以有任意字符 |
| _ | Used anywhere in a string to match a single character (only with LIKE or NOT LIKE) 和% 相似，代表1个字符 | col_name LIKE "AN_" (matches "AND", but not "AN") |
| IN (…) | String exists in a list 在列表 | col_name IN ("A", "B", "C") |
| NOT IN (…) | String does not exist in a list 不在列表 | col_name NOT IN ("D", "E", "F") |
ps：LIKE与“=”的区别在与，LIKE用于模糊匹配，例：
找到所有电影名为 "WALL-" 开头的电影 select * from movies where like"WALL-%"
查询结果Filtering过滤 和 sorting排序:
关键词：去重(去除重复的数据）（distinct) 句式：SELECT DISTINCT column, another_column, … FROM mytable WHERE condition(s)
排序（排列顺序） （order by) 句式：SELECT column, another_column, … FROM mytable WHERE condition(s) ORDER BY column ASC/DESC;
选取结果： （limit返回多少行结果，offset从哪开始剪）SELECT column, another_column, … FROM mytable WHERE condition(s) ORDER BY column ASC/DESC LIMIT num_limit OFFSET num_offset（例：按电影名字母序升序排列，列出前5部电影，select * from movies order by title limit 5 offset 0)
