总结
语法
select
查询列表
from
表
【where
筛选条件】
group by
分组的字段
having
分组后筛选
【order by
排序的字段】;
执行顺序
- from子句
- where子句
- group by子句
- having子句
- select子句
- order by子句
特点
- 和分组函数一同查询的字段必须是group by后出现的字段
- 筛选分为两类:分组前筛选和分组后筛选
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 |
表的结构
数据文件参见:
1. 简单的分组
案例1:查询每个工种的员工平均工资
SELECT job_id, AVG(salary)
FROM employees
GROUP BY job_id
案例2:查询每个位置的部门个数
SELECT COUNT(*),manager_id
from employees
WHERE manager_id is not null
GROUP BY manager_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) as 最高工资,department_id
FROM employees
where 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) as 平均工资,manager_id
from employees
where commission_pct is not null
GROUP 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
#② 筛选刚才①结果
SELECT department_id,COUNT(*) as "员工个数"
FROM employees
GROUP BY department_id
HAVING count(*)>5
注意是先找出每个部门的员工个数,再筛选出大于5的。所以是分组后筛选,要用HAVING(而不是WHERE),放在GROUP BY 的后面
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 employees
where commission_pct is not null
GROUP BY job_id
HAVING 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
select manager_id, min(salary)
FROM employees
WHERE manager_id>102
GROUP BY manager_id
having 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)
FROM employees
WHERE commission_pct is not null
GROUP BY job_id
HAVING MAX(salary)>6000
ORDER BY MAX(salary) ASC
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 job_id,department_id, MIN(salary)
from employees
GROUP BY job_id,department_id
ORDER BY MIN(salary) DESC