子查询中还可以包含比较运算符，如=、!=、>、<等。
-- 查询平均年龄在20以上的部门名称 select name from department where id in ( select dep_id from employee group by dep_id having avg(age) > 20); -- 查询财务部员工姓名 select name from employee where dep_id in ( select id from department where name=\'财务部\'); -- 查询所有大于平均年龄的员工的年龄和姓名 select name,age from employee where age > ( select avg(age) from employee);