MySQL 分组查询-GROUP BY

分组查询语法
select 分组函数，列
from 表名
【where】 筛选条件 -- where的位置可以放在group by的前面或者后面，两者有不同含义
group by 分组的列表
【having】筛选条件
【order by】 --如果使用到排序，order by语句基本在最后

where的使用特点：1，
数据源 位置 关键字
分组前筛选 来源于‘from’表中 group by前 where
分组后筛选 来源于分组函数计算过结果集 group by后 having

2，group by 子句支持单个字段，多个字段（字段之间用，隔开），表达式或者函数

SELECT MAX(salary),job_id
FROM employees
GROUP BY job_id;

SELECT COUNT(*),location_id
FROM departments
GROUP BY location_id;

添加筛选条件

SELECT AVG(salary),department_id
FROM employees
WHERE email LIKE '%a%'
GROUP BY department_id;

SELECT MAX(salary),manager_id
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY manager_id;

添加复杂的筛选条件

SELECT COUNT(*),department_id
FROM employees
GROUP BY department_id;

SELECT job_id,MAX(salary),commission_pct
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY job_id
HAVING MAX(salary)>12000;

按表达式或函数分组

SELECT COUNT(),LENGTH(last_name) len_name
FROM employees
GROUP BY LENGTH(last_name)
HAVING COUNT()>5;

按多个字段分组

SELECT AVG(salary),job_id,department_id
FROM employees
GROUP BY job_id,department_id;

添加排序

SELECT AVG(salary),job_id,department_id
FROM employees
GROUP BY job_id,department_id
ORDER BY AVG(salary) DESC;