一,
引入:公司里面有很多部门需要将相同部门编号的员工放在一组计算它的平局工资
语法:
select 分组函数 ,列(要求出现在group by 的后面)
from 表
【where 筛选条件】
group by 分组的列表
【order by 子句】
特点:
1,分组查询中的筛选条件分两类
数据源 位置
分组前查询 原始表 group by子句前面
分组后查询 分组后的结果集 group by子句后面
分组函数做条件肯定是放在having子句中
能用分组前筛选的,就分组前筛选
2,group by 支持子句支持单个字段分组,多个字段分组(多个字段之间用逗号隔开,没有顺序要求),表达式支持别名
3,也可以添加排序(排序放在整个分组查询的最后)
二:
1,案列1:简单分组查询
select max(salary), job_id
from employee
order by job_id;
2,案列2:查询每个位置上的部门个数
select count(*),location_id
from departments
order by location_id;
3,添加条件:查询邮箱中包含字段a字符的,每个部门的平均工资
select avg(salary) , department_id
from employees
where email like '%a%'
group by department_id;
4,查询有奖金的每个领导手下员工的最高工资
select max(salary) ,manager_id
from employees
where bonous is not null
group by manager_id;
5,查询那个部门的员工个数大于2——分组后的筛选
select count(*), department_id
from employees
group by department_id
having count(*)
6,查询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 max(salary) ,job_id
from employees
where bouns is not null
group by job_id
having max(salary)>12000;
7,查询领导编号》102的每个领导手下的员工最低工资>5000的领导编号号是哪个,
select min(salary),manager_id
from employees
where manage>102
order by manager_id
having min(salary)>5000.;
8,按员工姓名长度分组,查询每一组员工的个数,筛选员工个数>5的有哪些
select count(*),length(last_name) as len_name
from employees
group by length(last_name)
having count(*)>5;
9,按多个字段分组
案例:查询每个部门,每个工种的平局工资
select avg(salary),department_id ,job_id
from employees
group by job_id, department_id;
10,添加排序
案例:查询每个部门,每个工种的平局工资,按平均工资降序,部门id不为空的,且平均工资>10000的显示
select avg(salary),department_id ,job_id
where department_id is not null
from employees
group by job_id, department_id
having avg(salary)>10000
order by avg(salary) desc;