MySQL - 子查询

又称为内查询，外部的查询语句称为主查询或外查询

分类

1. 按位置
   1. select后
   2. from后
   3. where或having后
   4. exists后
2. 按结果集
   1. 标量子查询：一行一列
   2. 列子查询：多行一列
   3. 行子查询：一行多列
   4. 表子查询：任意，一般为多行多列

where或having后面

支持标量子查询、列子查询和行子查询

特点

1. 子查询语句放在小括号内
2. 子查询一般放在条件右侧
3. 标量子查询一般搭配单行操作符：> < >= <= <>；列子查询一般搭配多行操作符：in any/some all

标量子查询

谁的工资比 Abel 高

逐步分析

一、查询abel的工资

select salary
from employees
where last_name = 'Abel';

二、查询工资大于1结果的员工

select *
from employees
where salary > (
    select salary
    from employees
    where last_name = 'Abel'
);

返回job_id与141号员工相同，salary比143号员工多的员工 姓名，job_id 和工资

逐步分析

一、查询141号员工的job_id

select job_id
from employees
where employee_id = 141;

二、查询143号员工的salary

select salary
from employees
where employee_id = 143;

三、结合1和2结果查询

select last_name, job_id, salary
from employees
where job_id = (
    select job_id
    from employees
    where employee_id = 141
)
  and salary > (
    select salary
    from employees
    where employee_id = 143
);

返回公司工资最少的员工的last_name,job_id和salary

逐步分析

一、查询公司最低的工资

select min(salary) min
from employees;

二、结合结果1查询

select last_name, job_id, salary
from employees
where salary = (
    select min(salary) min
    from employees
);

查询最低工资大于50号部门最低工资的部门id和其最低工资

逐步分析

一、查询50号部门的最低工资

select min(salary)
from employees
where department_id = 50;

二、结合结果1查询

select department_id, min(salary) min
from employees
group by department_id
having min > (
    select min(salary)
    from employees
    where department_id = 50
);

列子查询

返回location_id是1400或1700的部门中的所有员工姓名

逐步分析

一、查询location_id是1400或1700的部门id

select distinct department_id
from departments
where location_id in (1400, 1700);

二、查询在这些部门id的员工姓名

select last_name
from employees
where department_id in (
    select distinct department_id
    from departments
    where location_id in (1400, 1700)
);

或者使用any关键字

select last_name
from employees
where department_id = any (
    select distinct department_id
    from departments
    where location_id in (1400, 1700)
);

返回其它工种中比job_id为‘IT_PROG’工种任一工资低的员工的员工号、姓名、job_id 以及salary

# 1
select distinct salary
from employees
where job_id = 'IT_PROG';

# 2
select employee_id, last_name, job_id, salary
from employees
where job_id <> 'IT_PROG'
  and salary < any (
    select distinct salary
    from employees
    where job_id = 'IT_PROG'
);

或者使用max关键字代替

select employee_id, last_name, job_id, salary
from employees
where job_id <> 'IT_PROG'
  and salary < (
    select max(salary)
    from employees
    where job_id = 'IT_PROG'
);

返回其它部门中比job_id为‘IT_PROG’部门所有员工工资都低的员工的员工号、姓名、job_id 以及salary

# 1
select distinct department_id
from employees
where job_id = 'IT_PROG';
# 2
select distinct salary
from employees
where department_id = (
    select distinct department_id
    from employees
    where job_id = 'IT_PROG'
);
# 3
select employee_id, last_name, job_id, salary
from employees
where department_id <> (
    select distinct department_id
    from employees
    where job_id = 'IT_PROG'
)
  and (
        salary < all (select distinct salary
                      from employees
                      where department_id = (
                          select distinct department_id
                          from employees
                          where job_id = 'IT_PROG'
                      ))
    );

行子查询

查询员工编号最小并且工资最高的员工信息

方法一：标量子查询方法

# 1
select min(employee_id)
from employees;
# 2
select max(salary)
from employees;
# 3
select *
from employees
where employee_id = (
    select min(employee_id)
    from employees
)
  and salary = (
    select max(salary)
    from employees
);

方法二：行子查询方法

select *
from employees
where (employee_id, salary) = (
    select min(employee_id), max(salary)
    from employees
);

select后面

仅支持标量子查询

查询每个部门的员工个数

select d.department_id,
       (
           select count(*)
           from employees e
           where e.department_id = d.department_id
       ) count
from departments d;

查询员工号=102的部门名

select (select department_name from departments d where d.department_id = e.department_id) d_name
from employees e
where e.employee_id = 102;

from后面

表子查询，将子查询结果充当一张表，要求必须取别名

查询每个部门的平均工资的工资等级

# 1
select department_id, avg(salary) avg
from employees
group by department_id;
# 2
select d.department_id did, d.avg avg, j.grade_level
from (select department_id, avg(salary) avg
      from employees
      group by department_id) d
         left join job_grades j on d.avg between j.lowest_sal and j.highest_sal;

exists后面

语法

select exists(查询语句);

判断结果集是否存在记录

结果为0或1

查询有员工的部门名

select department_name
from departments d
where exists(
              select * from employees e where e.department_id = d.department_id
          );