exists后面(相关子查询)

exists的作用是：判断子查询有没有结果的存在

案例：

select exists(select employee_id from employees); 返回的结果:1;

语法：

select exists(完整的子查询); 子查询有结果返回1,没有结果返回0;

案例：查询有员工的部门名

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

注意：能用exists的绝对能用前面讲过的in来实现,所以exists很少使用

select department_name from departments d
where d.department_id in (
select department_id from employees
);