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语法：
select 查询列表from 表【where 筛选条件】group by 分组的字段【order by 排序的字段】;
特点：
| 针对的表 | 位置 | 连接的关键字 | |
|---|---|---|---|
| 分组前筛选 | 原始表 | group by前 | where |
| 分组后筛选 | group by后的结果集 | group by后 | having |
问题1：分组函数做筛选能不能放在where后面
答：不能问题2：where——group by——having
一般来讲，能用分组前筛选的，尽量使用分组前筛选，提高效率
3、分组可以按单个字段也可以按多个字段
4、可以搭配着排序使用示例
#引入：查询每个部门的员工个数SELECT COUNT(*) FROM employees WHERE department_id=90;#1.简单的分组#案例1：查询每个工种的员工平均工资SELECT AVG(salary),job_idFROM employeesGROUP BY job_id;#案例2：查询每个位置的部门个数SELECT COUNT(*),location_idFROM departmentsGROUP BY location_id;#2、可以实现分组前的筛选#案例1：查询邮箱中包含a字符的 每个部门的最高工资SELECT MAX(salary),department_idFROM employeesWHERE email LIKE '%a%'GROUP BY department_id;#案例2：查询有奖金的每个领导手下员工的平均工资SELECT AVG(salary),manager_idFROM employeesWHERE commission_pct IS NOT NULLGROUP BY manager_id;#3、分组后筛选#案例：查询哪个部门的员工个数>5#①查询每个部门的员工个数SELECT COUNT(*),department_idFROM employeesGROUP BY department_id;#② 筛选刚才①结果SELECT COUNT(*),department_idFROM employeesGROUP BY department_idHAVING COUNT(*)>5;#案例2：每个工种有奖金的员工的最高工资>12000的工种编号和最高工资SELECT job_id,MAX(salary)FROM employeesWHERE commission_pct IS NOT NULLGROUP BY job_idHAVING MAX(salary)>12000;#案例3：领导编号>102的每个领导手下的最低工资大于5000的领导编号和最低工资manager_id>102SELECT manager_id,MIN(salary)FROM employeesGROUP BY manager_idHAVING MIN(salary)>5000;#4.添加排序#案例：每个工种有奖金的员工的最高工资>6000的工种编号和最高工资,按最高工资升序SELECT job_id,MAX(salary) mFROM employeesWHERE commission_pct IS NOT NULLGROUP BY job_idHAVING m>6000ORDER BY m ;#5.按多个字段分组#案例：查询每个工种每个部门的最低工资,并按最低工资降序SELECT MIN(salary),job_id,department_idFROM employeesGROUP BY department_id,job_idORDER BY MIN(salary) DESC;
#1.查询各job_id的员工工资的最大值，最小值，平均值，总和，并按job_id升序SELECT MAX(salary),MIN(salary),AVG(salary),SUM(salary),job_idFROM employeesGROUP BY job_idORDER BY job_id;#2.查询员工最高工资和最低工资的差距（DIFFERENCE）SELECT MAX(salary)-MIN(salary) DIFFRENCEFROM employees;#3.查询各个管理者手下员工的最低工资，其中最低工资不能低于6000，没有管理者的员工不计算在内SELECT MIN(salary),manager_idFROM employeesWHERE manager_id IS NOT NULLGROUP BY manager_idHAVING MIN(salary)>=6000;#4.查询所有部门的编号，员工数量和工资平均值,并按平均工资降序SELECT department_id,COUNT(*),AVG(salary) aFROM employeesGROUP BY department_idORDER BY a DESC;#5.选择具有各个job_id的员工人数SELECT COUNT(*) 个数,job_idFROM employeesGROUP BY job_id;
#1.查询员工的姓名和部门号和年薪，按年薪降序 按姓名升序SELECT last_name,department_id,salary*12*(1+IFNULL(commission_pct,0)) 年薪FROM employeesORDER BY 年薪 DESC,last_name ASC;#2.选择工资不在8000到17000的员工的姓名和工资，按工资降序SELECT last_name,salaryFROM employeesWHERE salary NOT BETWEEN 8000 AND 17000ORDER BY salary DESC;#3.查询邮箱中包含e的员工信息，并先按邮箱的字节数降序，再按部门号升序SELECT *,LENGTH(email)FROM employeesWHERE email LIKE '%e%'ORDER BY LENGTH(email) DESC,department_id ASC;
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