用exist 当作布尔值来处理,当查询语句有结果时 返回true, 否则返回false.
1 select * from emp where exists (select * from emp where neo=123);
1 --查询有员工的部门的信息 2 3 select * from dept d1 where exists(select * from emp e1 where e1.deptno = d1.deptno );
1 --查询不是领导的信息(含null值正确写法) 2 3 select * from emp where empno not in (select mgr from emp where mgr is not null); --查询出8条记录