mysql 按表达式或函数分组、多个字段分组、排序

按表达式或函数分组：

#案例：按员工的姓名的长度分组，查询每一组的员工个数，筛选员工个数大于五

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

按多个字段分组：
#案例：查询每个部门每个工种的员工的平均工资

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

排序
#案例：查询每个部门每个工种的部门编号不为null的员工的平均工资>10000，并且按平均工资的高低显示

SELECT     AVG(salary),department_id,job_id FROM     employees WHERE     department_id IS NOT NULL GROUP BY     department_id,job_id HAVING     AVG(salary)>10000 ORDER BY     AVG(salary) DESC;

来源：51CTO

作者：wx5d21d5e6e5ab1

链接：https://blog.51cto.com/14437184/2436653