子查询：
凡是需要分步进行而后几步需要依赖前几步时，就需要用到子查询。
分布进行时先计算子查询。
可以把子查询当做函数。
练习：
1.工资大于全公司平均工资的员工姓名。
select last_name,salary
from employees
where salary>
(select avg(salary) from employees); //先查询出全公司平均员工作为子查询
2.在Seattle工作的所有员工姓名
select last_name
from employees
where department_id in
(select department_id from departments
where location_id=
(select location_id from locations where city='Seattle'));
配对子查询：
练习：和Feeney在同一个部门、做同一职位的员工姓名：
select last_name, department_id, job_id
from employees
where (department_id, job_id)=
(select department_id, job_id from employees where last_name='Feeney')
and last_name != 'Feeney';