子查詢 　1、單行子查詢： 　　  例如：select * from emp where sal>(select sal from emp where ename = 'ALLEN'); 　2、多行子查詢：any all in 　　 　　大於any就是大於最小值 　　select * from emp where sal>any(select sal from emp where deptno=10); 　　select * from emp where sal>(select min(sal) from emp where deptno=10); 　　　　大於all就是大於最大值 　　select * from emp where sal>all(select sal from emp where deptno=10); 　　select * from emp where sal>(select max(sal) from emp where deptno=10); 　　　　小於any就是小於最大值 　　select * from emp where sal<any(select sal from emp where deptno=10); 　　select * from emp where sal<(select max(sal) from emp where deptno=10); 　　　　小於all就是小於最小值 　　select * from emp where sal<all(select sal from emp where deptno=10); 　　select * from emp where sal<(select min(sal) from emp where deptno=10); in (用法舉例如下) 3、多列子查詢：（瞭解） 　　select deptno , job from emp where deptno != 10; 　　select * from emp where (deptno,job) in (select deptno , job from emp where deptno != 10); 4、相關子查詢： 　　每執行一次主查詢，就要執行一次子查詢。 　　查詢各個部門工資比平均工資高的人的資訊 select * from emp where deptno = 10 and sal > (select avg(sal) from emp where deptno=10); select * from emp where deptno = 20 and sal > (select avg(sal) from emp where deptno=20); select * from emp where deptno = 30 and sal > (select avg(sal) from emp where deptno=30); select * from emp e1 where sal > (select avg(sal) from emp e2 where e1.deptno = e2.deptno);   5、練習題截圖：