数据库 基础笔记5

#进阶五:分组查询
/*
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
       order by
    `set_time`
          子句
   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                       后面        having
      
      注意:
      1,分组函数做筛选条件肯定是放在having字句中
      2，能用分组前筛选，就用分组前筛选
      3,group by 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），表达式或则函数
      4,也可以添加排序，（排序放在整个语句的结尾）
*/
#查询每一个部门的平均工资
SELECT
      TRUNCATE(AVG(salary),2),department_id
FROM
   employees
GROUP BY
       department_id;
#---------------------
#查询每一个工种的最高工资并升序排列
SELECT
      MAX(salary),job_id
FROM
     employees
GROUP BY
       job_id
ORDER BY
      salary;
#-------------------------
#查询每个位置上的部门个数
SELECT
     COUNT(*),
     location_id
FROM
    departments
GROUP BY
       location_id;
#分组前添加筛选条件      
#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;
#---------------------------------------
#查询有奖金的每一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;
#---------------------------------------
#分组后添加筛选条件
#查询哪个部门的员工个数 > 2
#思路：1，先查询每一部门员工个数，2，再筛选个数大于2的部门
SELECT
     COUNT(*),
     department_id
FROM 
    employees
GROUP BY
       department_id
HAVING
     COUNT(*) > 2;
#分组前后筛选条件搭配使用
#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;
#---------------------------------------------------------
#查询领导编号 > 102的每个领导手下的最低工资 > 5000 的领导编号及其最低工资
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
#按员工姓名长度分组，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：查询每一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;
       
#-----------------------------------------------
#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地显示
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;
 #-------------------------------------------案例-
 #1,查询各个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
 DESC;
 
#2,查询员工最高工资和最低工资的差距
SELECT
      MAX(salary) - MIN(salary) AS 差距
FROM
    employees;
    
#-------------------
#3,查询各个管理者手下的员工的最低工资，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，员工数量和平均工资，并按平均工资降序
SELECT
     
     COUNT(*),
     AVG(salary),
     department_id
FROM
    employees
GROUP BY
       department_id
 ORDER BY
        AVG(salary)
 DESC;
 #------------------
 #查询每一个job_id的员工人数
 SELECT
       COUNT(*),
       job_id
 FROM
      employees
 GROUP BY
        job_id;