MySQL之单表查询
一，语法概述
1、单表查询的语法
- SELECT 字段1,字段2... FROM 表名
- WHERE 条件
- GROUP BY field
- HAVING 筛选
- ORDER BY field
- LIMIT 限制条数
2、关键字的执行优先级
重点中的重点：关键字的执行优先级
- from
- where
- group by
- having
- select
- distinct
- order by
- limit
- 找到表:from
- 拿着where指定的约束条件，去文件/表中取出一条条记录
- 将取出的一条条记录进行分组group by，如果没有group by，则整体作为一组
- 将分组的结果进行having过滤
- 执行select
- 去重
- 将结果按条件排序：order by
- 限制结果的显示条数
ex:创建公司员工表，表的字段和数据类型
#创建表,设置字段的约束条件 create table employee( id int primary key auto_increment, name varchar(20) not null, sex enum('male','female') not null default 'male', #大部分是男的 age int(3) unsigned not null default 28, hire_date date not null, post varchar(50), post_comment varchar(100), salary double(15,2), office int,#一个部门一个屋 depart_id int ); # 查看表结构 #插入记录 #三个部门：教学，销售，运营 insert into employee(name ,sex,age,hire_date,post,salary,office,depart_id) values ('long','male',18,'20170301','大使',7300.33,401,1), #以下是教学部 ('gou','male',78,'20150302','teacher',1000000.31,401,1), ('hui','male',81,'20130305','teacher',8300,401,1), ('yuanhao','male',73,'20140701','teacher',3500,401,1), ('liwenzhou','male',28,'20121101','teacher',2100,401,1), ('jingliyang','female',18,'20110211','teacher',9000,401,1), ('jinxin','male',18,'19000301','teacher',30000,401,1), ('xiaomage','male',48,'20101111','teacher',10000,401,1), ('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门 ('丫丫','female',38,'20101101','sale',2000.35,402,2), ('丁丁','female',18,'20110312','sale',1000.37,402,2), ('星星','female',18,'20160513','sale',3000.29,402,2), ('格格','female',28,'20170127','sale',4000.33,402,2), ('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门 ('程咬金','male',18,'19970312','operation',20000,403,3), ('程咬银','female',18,'20130311','operation',19000,403,3), ('程咬铜','male',18,'20150411','operation',18000,403,3), ('程咬铁','female',18,'20140512','operation',17000,403,3) ;
（1）where 约束
where子句中可以使用 1.比较运算符：>、<、>=、<=、<>、!= 2.between 80 and 100 ：值在80到100之间 3.in(80,90,100)值是10或20或30 4.like 'xiaomagepattern': pattern可以是%或者_。%小时任意多字符，_表示一个字符 5.逻辑运算符：在多个条件直接可以使用逻辑运算符 and or not
#1 :单条件查询 mysql> select id,emp_name from employee where id > 5; +----+------------+ | id | emp_name | +----+------------+ | 6 | jingliyang | | 7 | jinxin | | 8 | xiaomage | | 9 | 歪歪 | | 10 | 丫丫 | | 11 | 丁丁 | | 12 | 星星 | | 13 | 格格 | | 14 | 张野 | | 15 | 程咬金 | | 16 | 程咬银 | | 17 | 程咬铜 | | 18 | 程咬铁 | #2 多条件查询 mysql> select emp_name from employee where post='teacher' and salary>10000; +----------+ | emp_name | +----------+ | alex | | jinxin | +----------+ #3.关键字BETWEEN AND SELECT name,salary FROM employee WHERE salary BETWEEN 10000 AND 20000; SELECT name,salary FROM employee WHERE salary NOT BETWEEN 10000 AND 20000; #注意''是空字符串，不是null SELECT name,post_comment FROM employee WHERE post_comment=''; ps： 执行 update employee set post_comment='' where id=2; 再用上条查看，就会有结果了 #5:关键字IN集合查询 mysql> SELECT name,salary FROM employee WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ; +------------+---------+ | name | salary | +------------+---------+ | yuanhao | 3500.00 | | jingliyang | 9000.00 | +------------+---------+ rows in set (0.00 sec) mysql> SELECT name,salary FROM employee WHERE salary IN (3000,3500,4000,9000) ; +------------+---------+ | name | salary | +------------+---------+ | yuanhao | 3500.00 | | jingliyang | 9000.00 | +------------+---------+ mysql> SELECT name,salary FROM employee WHERE salary NOT IN (3000,3500,4000,9000) ; +-----------+------------+ | name | salary | +-----------+------------+ | egon | 7300.33 | | alex | 1000000.31 | | wupeiqi | 8300.00 | | liwenzhou | 2100.00 | | jinxin | 30000.00 | | xiaomage | 10000.00 | | 歪歪 | 3000.13 | | 丫丫 | 2000.35 | | 丁丁 | 1000.37 | | 星星 | 3000.29 | | 格格 | 4000.33 | | 张野 | 10000.13 | | 程咬金 | 20000.00 | | 程咬银 | 19000.00 | | 程咬铜 | 18000.00 | | 程咬铁 | 17000.00 | +-----------+------------+ rows in set (0.00 sec) #6:关键字LIKE模糊查询 通配符’%’ mysql> SELECT * FROM employee WHERE name LIKE 'jin%'; +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+ | 6 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 | | 7 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 | +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+ rows in set (0.00 sec) 通配符'_' mysql> SELECT age FROM employee WHERE name LIKE 'ale_'; +-----+ | age | +-----+ | 78 | +-----+ row in set (0.00 sec) 练习： 1. 查看岗位是teacher的员工姓名、年龄 2. 查看岗位是teacher且年龄大于30岁的员工姓名、年龄 3. 查看岗位是teacher且薪资在9000-1000范围内的员工姓名、年龄、薪资 4. 查看岗位描述不为NULL的员工信息 5. 查看岗位是teacher且薪资是10000或9000或30000的员工姓名、年龄、薪资 6. 查看岗位是teacher且薪资不是10000或9000或30000的员工姓名、年龄、薪资 7. 查看岗位是teacher且名字是jin开头的员工姓名、年薪 #对应的sql语句 select name,age from employee where post = 'teacher'; select name,age from employee where post='teacher' and age > 30; select name,age,salary from employee where post='teacher' and salary between 9000 and 10000; select * from employee where post_comment is not null; select name,age,salary from employee where post='teacher' and salary in (10000,9000,30000); select name,age,salary from employee where post='teacher' and salary not in (10000,9000,30000); select name,salary*12 from employee where post='teacher' and name like 'jin%'; where约束
（2）group by 分组查询
#1、首先明确一点：分组发生在where之后，即分组是基于where之后得到的记录而进行的 #2、分组指的是：将所有记录按照某个相同字段进行归类，比如针对员工信息表的职位分组，或者按照性别进行分组等 #3、为何要分组呢？ 取每个部门的最高工资 取每个部门的员工数 取男人数和女人数 小窍门：‘每’这个字后面的字段，就是我们分组的依据 #4、大前提： 可以按照任意字段分组，但是分组完毕后，比如group by post，只能查看post字段，如果想查看组内信息，需要借助于聚合函数
当执行以下sql语句的时候，是以post字段查询了组中的第一条数据，没有任何意义，因为我们现在想查出当前组的多条记录。
mysql> select * from employee group by post; +----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ | 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 | | 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 | | 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 | | 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 | +----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ rows in set (0.00 sec) #由于没有设置ONLY_FULL_GROUP_BY,于是也可以有结果，默认都是组内的第一条记录，但其实这是没有意义的 如果想分组，则必须要设置全局的sql的模式为ONLY_FULL_GROUP_BY mysql> set global sql_mode='ONLY_FULL_GROUP_BY'; Query OK, 0 rows affected (0.00 sec) #查看MySQL 5.7默认的sql_mode如下： mysql> select @@global.sql_mode; +--------------------+ | @@global.sql_mode | +--------------------+ | ONLY_FULL_GROUP_BY | +--------------------+ row in set (0.00 sec) mysql> exit;#设置成功后，一定要退出，然后重新登录方可生效 Bye
继续验证通过group by分组之后，只能查看当前字段,如果想查看组内信息，需要借助于聚合函数
mysql> select * from emp group by post;# 报错 ERROR 1054 (42S22): Unknown column 'post' in 'group statement' mysql> select post from employee group by post; +-----------------------------------------+ | post | +-----------------------------------------+ | operation | | sale | | teacher | | 大使 | +-----------------------------------------+ rows in set (0.00 sec)
（3）聚合函数
max()求最大值 min()求最小值 avg()求平均值 sum() 求和 count() 求总个数 #强调：聚合函数聚合的是组的内容，若是没有分组，则默认一组 # 每个部门有多少个员工 select post,count(id) from employee group by post; # 每个部门的最高薪水 select post,max(salary) from employee group by post; # 每个部门的最低薪水 select post,min(salary) from employee group by post; # 每个部门的平均薪水 select post,avg(salary) from employee group by post; # 每个部门的所有薪水 select post,sum(age) from employee group by post;
1. 查询岗位名以及岗位包含的所有员工名字 select post group_concat(name) from employee group by post; 2. 查询岗位名以及各岗位内包含的员工个数 mysql> select post,count(id) from employee group by post; +-----------------------------------------+-----------+ | post | count(id) | +-----------------------------------------+-----------+ | operation | 5 | | sale | 5 | | teacher | 7 | | 老男孩驻沙河办事处外交大使 | 1 | +-----------------------------------------+-----------+ 3. 查询公司内男员工和女员工的个数 mysql> select sex,count(1) from employee group by sex; +--------+----------+ | sex | count(1) | +--------+----------+ | male | 10 | | female | 8 | +--------+----------+ 2 rows in set (0.00 sec) 4. 查询岗位名以及各岗位的平均薪资 mysql> select post,avg(salary) from employee group by post; +-----------------------------------------+---------------+ | post | avg(salary) | +-----------------------------------------+---------------+ | operation | 16800.026000 | | sale | 2600.294000 | | teacher | 151842.901429 | | 老男孩驻沙河办事处外交大使 | 7300.330000 | +-----------------------------------------+---------------+ 4 rows in set (0.00 sec) 5. 查询岗位名以及各岗位的最高薪资 6. 查询岗位名以及各岗位的最低薪资 7. 查询男员工与男员工的平均薪资，女员工与女员工的平均薪资 mysql> select sex,avg(salary) from employee group by sex; +--------+---------------+ | sex | avg(salary) | +--------+---------------+ | male | 110920.077000 | | female | 7250.183750 | +--------+---------------+ 2 rows in set (0.00 sec)
（4）HAVING过滤
HAVING与WHERE不一样的地方在于 #！！！执行优先级从高到低：where > group by > having #1. Where 发生在分组group by之前，因而Where中可以有任意字段，但是绝对不能使用聚合函数。 #2. Having发生在分组group by之后，因而Having中可以使用分组的字段，无法直接取到其他字段,可以使用聚合函数
验证： mysql> select * from employee where salary>1000000; +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ | 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 | +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ row in set (0.00 sec) mysql> select * from employee having salary>1000000; mysql> select * from employee having salary>1000000; +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ | 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 | +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ # 必须使用group by才能使用group_concat()函数，将所有的name值连接 mysql> select post,group_concat(name) from emp group by post having salary > 10000; ##错误，分组后无法直接取到salary字段 ERROR 1054 (42S22): Unknown column 'post' in 'field list'
1. 查询各岗位内包含的员工个数小于2的岗位名、岗位内包含员工名字、个数 mysql> select post,group_concat(name),count(1) from employee group by post having count(1)<2; +-----------------------------------------+--------------------+----------+ | post | group_concat(name) | count(1) | +-----------------------------------------+--------------------+----------+ | 大使 | egon | 1 | +-----------------------------------------+--------------------+----------+ 1 row in set (0.00 sec) 2. 查询各岗位平均薪资大于10000的岗位名、平均工资 mysql> select post,avg(salary) as ee from employee group by post having ee>10000; +-----------+---------------+ | post | ee | +-----------+---------------+ | operation | 16800.026000 | | teacher | 151842.901429 | +-----------+---------------+ 2 rows in set (0.00 sec) 3. 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资 mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) <20000; +-----------+--------------+ | post | avg(salary) | +-----------+--------------+ | operation | 16800.026000 | +-----------+--------------+ 1 row in set (0.00 sec)
（5）order by 查询排序
按单列排序 SELECT * FROM employee ORDER BY age; SELECT * FROM employee ORDER BY age ASC; SELECT * FROM employee ORDER BY age DESC; 按多列排序:先按照age升序排序，如果年纪相同，则按照id降序 SELECT * from employee ORDER BY age ASC, id DESC;
验证多列排序： SELECT * from employee ORDER BY age ASC,id DESC; mysql> SELECT * from employee ORDER BY age ASC,id DESC; +----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ | 18 | 程咬铁 | female | 18 | 2014-05-12 | operation | NULL | 17000.00 | 403 | 3 | | 17 | 程咬铜 | male | 18 | 2015-04-11 | operation | NULL | 18000.00 | 403 | 3 | | 16 | 程咬银 | female | 18 | 2013-03-11 | operation | NULL | 19000.00 | 403 | 3 | | 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 | | 12 | 星星 | female | 18 | 2016-05-13 | sale | NULL | 3000.29 | 402 | 2 | | 11 | 丁丁 | female | 18 | 2011-03-12 | sale | NULL | 1000.37 | 402 | 2 | | 7 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 | | 6 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 | | 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 | | 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 | | 13 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 | | 5 | liwenzhou | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 | | 10 | 丫丫 | female | 38 | 2010-11-01 | sale | NULL | 2000.35 | 402 | 2 | | 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 | | 8 | xiaomage | male | 48 | 2010-11-11 | teacher | NULL | 10000.00 | 401 | 1 | | 4 | yuanhao | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 | | 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 | | 3 | wupeiqi | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 | +----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ rows in set (0.01 sec) mysql>
1. 查询所有员工信息，先按照age升序排序，如果age相同则按照hire_date降序排序 mysql> select * from employee order by age asc,hire_date desc; 2. 查询各岗位平均薪资大于1000的岗位名、平均工资,结果按平均薪资升序排列 mysql> select post,avg(salary) from employee group by post having avg(salary>1000) order by avg(salary) asc; +-----------------------------------------+---------------+ | post | avg(salary) | +-----------------------------------------+---------------+ | sale | 2600.294000 | | 老男孩驻沙河办事处外交大使 | 7300.330000 | | operation | 16800.026000 | | teacher | 151842.901429 | +-----------------------------------------+---------------+ 4 rows in set (0.00 sec) 3. 查询各岗位平均薪资大于1000的岗位名、平均工资,结果按平均薪资降序排列 mysql> select post,avg(salary) from employee group by post having avg(salary>1000) order by avg(salary) desc; +-----------------------------------------+---------------+ | post | avg(salary) | +-----------------------------------------+---------------+ | teacher | 151842.901429 | | operation | 16800.026000 | | 老男孩驻沙河办事处外交大使 | 7300.330000 | | sale | 2600.294000 | +-----------------------------------------+---------------+ 4 rows in set (0.00 sec)
（6）limit 限制查询的记录数
示例： SELECT * FROM employee ORDER BY salary DESC LIMIT 3; #默认初始位置为0 SELECT * FROM employee ORDER BY salary DESC LIMIT 0,5; #从第0开始，即先查询出第一条，然后包含这一条在内往后查5条 SELECT * FROM employee ORDER BY salary DESC LIMIT 5,5; #从第5开始，即先查询出第6条，然后包含这一条在内往后查5条
# 第1页数据 mysql> select * from employee limit 0,5; +----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ | 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 | | 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 | | 3 | wupeiqi | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 | | 4 | yuanhao | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 | | 5 | liwenzhou | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 | +----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ rows in set (0.00 sec) # 第2页数据 mysql> select * from employee limit 5,5; +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+ | 6 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 | | 7 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 | | 8 | xiaomage | male | 48 | 2010-11-11 | teacher | NULL | 10000.00 | 401 | 1 | | 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 | | 10 | 丫丫 | female | 38 | 2010-11-01 | sale | NULL | 2000.35 | 402 | 2 | +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+ rows in set (0.00 sec) # 第3页数据 mysql> select * from employee limit 10,5; +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ | 11 | 丁丁 | female | 18 | 2011-03-12 | sale | NULL | 1000.37 | 402 | 2 | | 12 | 星星 | female | 18 | 2016-05-13 | sale | NULL | 3000.29 | 402 | 2 | | 13 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 | | 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 | | 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 | +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ rows in set (0.00 sec)
学无止境！
