7.分组查询

分组数据：GROUP BY 子句语法
功能：可以使用 group by 子句将表中的数据分成若干组
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 分组列表,[HAVING 分组后表达式]]
[ORDER BY 子句]
明确：WHERE 一定放在FROM后面
注意：查询列表必须特殊，要求是分组函数和group by后出现的字段
特点：
1、分组查询中的筛选条件分为两类
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放在having子句中。
② 能用分组前筛选，就优先考虑用分组前筛选。
2、group by 子句支持单个字段分组，也支持多个字段分组（多个字段之间用逗号隔开没有顺序要求），还支持表达式或函数（用的较少）。
3、也可以添加排序（排序放在分组查询语句之后）。

引入：查询每个部门的平均工资

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

案例1:查询每个工种的最高工资

SELECT MAX(salary),department_id 
FROM employees 
GROUP BY department_id 
ORDER BY MAX(salary) DESC;

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-分组前筛选

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`,
  `email`  
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最高工资

SELECT *,MAX(salary),`manager_id` 
FROM `employees` 
WHERE commission_pct IS NOT NULL 
GROUP  BY `manager_id`;

添加筛选条件-分组后筛选

案例1：查询那个部门的员工个数>2

SELECT  
  COUNT(*),
  `department_id`  
FROM
  `employees`  
GROUP BY `department_id`  
HAVING COUNT(*) > 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的领导编号是那个.

SELECT  
  MIN(salary),
  manager_id  
FROM
  `employees`  
WHERE `manager_id` > 102  
GROUP BY manager_id  
HAVING MIN(salary) > 5000 ;

按表达式或函数分组

案例1：按员工姓名的长度分组，查询每一组的员工个数，筛选员工个数>5的

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

案例2：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` ;

案例3：查询每个部门每个工种的员工的平均工资，并且排序

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 ;

五道小题

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_id  
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(别名DIFFERNCE)

SELECT MAX(salary)-MIN(salary) AS DIFFERNCE FROM `employees`;

3.查询各个管理者手下员工的最低工资，其中最低工不能低于6000，没有管理这的员工不计算在内

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，员工数量和工资平均值，并按平均工资降序

SELECT  
  `department_id`,
  COUNT(*),
  AVG(salary)  
FROM
  `employees`  
GROUP BY `department_id`  
ORDER BY AVG(salary) DESC;

5.选择具有各个job_id的员工人数

SELECT COUNT(*),job_id FROM `employees` GROUP BY job_id ORDER BY COUNT(*) ASC;