0 该博客所要用的数据库表的属性

1 分组函数

1.1 简单的使用

1. COUNT(expression)：计算符合指定条件的行数。常用于计算记录数量。

2. SUM(expression)：对指定列的值求和。通常用于计算数值型字段的总和。

3. AVG(expression)：计算指定列的平均值。常用于计算数值型字段的平均值。

4. MIN(expression)：计算指定列的最小值。用于找出某一列中的最小值。

5. MAX(expression)：计算指定列的最大值。用于找出某一列中的最大值。

select sum(salary) as 和,count(salary) as 个数,round(avg(salary),2) as 平均,max(salary) as 最高,min(salary) as 最低
from employees;

1.2 是否忽略null值

null+任意值=null

select sum(commission_pct),avg(commission_pct)
from employees;

1.1的分组函数都忽略null值

1.3 和关键字搭配使用

select count(distinct salary) as 有几种工资
from employees;

1.4 count函数的详细介绍

统计行数

select count(*)
from employees;

1.5 练习

1

selectmax(salary) as 最大值, min(salary) as 最小值, avg(salary) as 平均值, sum(salary) as 总和
from employees;

2

select DATEDIFF(max(hiredate),min(hiredate)) as DIFFERENCE
from employees;

3

select count(*) as 员工个数
from employees
where department_id=90;

2 分组查询Group by

select
from
where
group by
order by

2.1 简单的分组查询

1.查询每个工种的最高工资

select max(salary),job_id
from employees
group by job_id;

2.查询哪个部门的员工个数>2

select count(*),department_id
from employees
group by department_id
having count(*)>2;

3.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 byjob_id
having max(salary)>12000;

4.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，以及其最低工资

select manager_id,min(salary)
from employees
where manager_id>102
group by manager_id
having min(salary)>5000;

2.2 练习

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 job_id,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) as 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，没有管理者的员工不计算在内

select manager_id,min(salary)
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(*) as 员工数量,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 job_id,count(*) as 员工人数
from employees
group by job_id;

---

3 格式投票😋

题外话，有一个格式投票，大家认为哪种好看呢
格式1.

select employee_id
from employee;

格式2.

selectemployee_id
fromemployee;