#进阶五:分组查询
/*
语法:
select 分组函数,列(要求出现在group by后面)
from 表
【where 筛选条件】
group by 分组列表
【order by 子句】
注意:
查询列表必须特殊,要求分组函数和group by后出现的字段
特点:
1、分组查询分为两类
数据源 位置 关键字
分组前筛选 原始表 group by子句的前面 where
分组后筛选 分组后的结果集 group by子句的后面 having
①分组函数做条件肯定是having子句
②能用分组前筛选的尽量放到group by前面
2、group by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)
3、也可以排序(排序放在整个分组查询的最后)
*/
#引入:查询每个部门的平均工资
SELECT AVG(salary) FROM employees;#全部人的平均工资
#简单的分组前查询
#案例1:查询每个工种的最高工资
SELECT MAX(salary),job_id
FROM employees
GROUP BY job_id;
#案例2:查询每个位置上的部门个数
SELECT COUNT(*),location_id
FROM departments
GROUP BY location_id;
#添加筛选条件
#案例1:查询邮箱中包含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;
#案例2:查询每个领导手下有奖金的员工的最高工资
SELECT MAX(salary),manager_id
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY manager_id;
#添加分组后的筛选条件
#案例1:查询各部门的员工个数,且员工个数>2
#①查询每个部门的员工个数
SELECT COUNT(*) 员工个数,department_id
FROM employees
#where count(*)>2 不能这样写
GROUP BY department_id
#②根据①的结果进行筛选
HAVING 员工个数>2;
#案例2: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 commission_pct IS NOT NULL
GROUP BY job_id
HAVING MAX(salary)>12000;
#案例3:查询领导编号>102的每个领导手下的最低工资>5000的领导编号是哪个,以及最低工资
SELECT MIN(salary),manager_id
FROM employees
GROUP BY manager_id
HAVING manager_id>102 && MIN(salary)>5000;
#按表达式或函数分组
#案例:按员工的姓名的长度分组分组,查询每一组的员工个数,筛选员工个数>5的有哪些
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;
#添加排序
#案例:查询每个部门每个工种的员工的平均工资,并且按平均工资的高低显示
SELECT AVG(salary),department_id,job_id
FROM employees
GROUP BY department_id,job_id
ORDER BY AVG(salary) DESC;
#练习题:
#1.查询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)
FROM employees
GROUP BY job_id
ORDER BY job_id ASC;
#2.查询员工最高工资和最低工资的差距(difference)
SELECT MAX(salary)-MIN(salary) difference
FROM 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,没有管理者员工的不计算在内
SELECT MIN(salary),manager_id
FROM employees
WHERE manager_id IS NOT NULL
GROUP BY manager_id
HAVING MIN(salary)>=6000;
#4.查询所有部门的编号、员工个数和工资平均值,并按平均工资降序
SELECT department_id,COUNT(*),COUNT(*),AVG(salary)
FROM employees
GROUP BY department_id;
ORDER BY AVG(salary) DESC;
#5.选择具有各个job_id的员工个数
SELECT COUNT(*),job_id
FROM employees
GROUP BY job_id;