子查询

用法：

内部嵌套一个查询

案例：查询和zlotkey相同的员工姓名和工资

1.查询zlotkey的部门

select department_id 

from employees

where last_name = 'zlotkey'

2.查询部门号＝1的姓名和工资

select last_name,salary

from employees 

where department_id =(

　　select department_id 

　　from employees

　　where last_name = 'zlotkey'

)