MySQL--基础七（分组查询、排序查询）

墨蓝 2021-08-13 18:41 531阅读 0赞

#### 分组查询 ####

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：

1.  和分组函数一同查询的字段必须是group by后出现的字段
2.  筛选分为两类：分组前筛选和分组后筛选

<table> 
 <thead> 
  <tr> 
   <th align="left"></th> 
   <th align="left">针对的表</th> 
   <th align="left">位置</th> 
   <th align="left">连接的关键字</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left">分组前筛选</td> 
   <td align="left">原始表</td> 
   <td align="left">group by前</td> 
   <td align="left">where</td> 
  </tr> 
  <tr> 
   <td align="left">分组后筛选</td> 
   <td align="left">group by后的结果集</td> 
   <td align="left">group by后</td> 
   <td align="left">having</td> 
  </tr> 
 </tbody> 
</table>

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_id
    FROM employees
    GROUP BY job_id;
    
    #案例2：查询每个位置的部门个数
    
    SELECT COUNT(*),location_id
    FROM departments
    GROUP 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_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),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
    SELECT COUNT(*),department_id
    FROM employees
    GROUP BY department_id;
    
    #② 筛选刚才①结果
    
    SELECT COUNT(*),department_id
    FROM employees
    
    GROUP BY department_id
    
    HAVING 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 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
    
    manager_id>102
    
    SELECT manager_id,MIN(salary)
    FROM employees
    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) m
    FROM employees
    WHERE commission_pct IS NOT NULL
    GROUP BY job_id
    HAVING m>6000
    ORDER 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_id
    FROM employees
    GROUP BY department_id,job_id
    ORDER 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_id
    FROM employees
    GROUP BY job_id
    ORDER 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) DIFFRENCE
    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) a
    FROM employees
    GROUP BY department_id
    ORDER BY a DESC;
    #5.选择具有各个job_id的员工人数
    SELECT COUNT(*) 个数,job_id
    FROM employees
    GROUP 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 employees
    ORDER 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,salary
    FROM employees
    
    WHERE salary NOT BETWEEN 8000 AND 17000
    ORDER 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 employees
    WHERE email LIKE '%e%'
    ORDER BY LENGTH(email) DESC,department_id ASC;