查询本部门最高工资的员工信息：emp：员工信息表 deptno:部门号 sal:员工薪水
不相关其中几条：
select * from emp e where e.deptno=10 and sal =(select max(sal) from emp where deptno=10)
unino
select * from emp e where e.deptno=20 and sal =(select max(sal) from emp where deptno=20)
union
...
转换：相关子查询
select * from emp e where sal =(select max(sal) from emp where deptno=e.deptno)
子查询不能单独运行：先运行外查询有了e表才能运行子查询