二、放在select后面的子查询 仅仅支持标量子查询
案例1：查询每个部门的员工个数
select d.*,(select count(1) from employees e where e.department_id=d.department_id ) from departments d;--27行 有的部门是没有员工的显示为0
select d.department_id,count(1) from employees e,departments d where e.department_id=d.department_id group by d.department_id--11行 所有的部门都是有员工的
案例2：查询员工号码=102的部门名
select (select d.department_name from employees e INNER join departments d on e.department_id=d.department_id where e.employee_id='102') 部门名;
三、放在from后面的select语句
将查询结果充当一张表，必须起别名
案例1：查询每个部门的平均工资的工资等级
步骤一：查询每个部门的平均工资
select avg(salary),department_id from employees GROUP BY department_id;
步骤二：工资等级
select * from job_grades;
步骤三：整体结合
select ag_dep.*,g.grade_level
from (select avg(salary) ag,department_id from employees GROUP BY department_id) ag_dep
INNER JOIN job_grades g
on ag_dep.ag BETWEEN lowest_sal and highest_sal;
