子查询

子查询实例展示：

1.如何查的所有比“CLARK”工资高的员工的信息？

select *  from emp 
where sal > (
            select sal from emp 
            where ename = 'CLARK'
);

2. 如何查询工资高于平均工资的员工的姓名和工资？

select ename, sal 
from emp
where sal > (
        select avg(sal) from emp
);

3. 思考：查询scott同一个部门并且比他工资低的员工的名字和工资？

SELECT ENAME,SAL 
FROM EMP 
WHERE  SAL<=(select sal from emp where ename = 'SCOTT')
        AND 
       DEPTNO = (SELECT DEPTNO FROM EMP WHERE ENAME='SCOTT'
); 

 单行子查询：

4. 如何查询职务和scott相同，比scott雇佣时间早的员工信息

select * from emp
where job = (select job from emp where ename ='SCOTT')
        AND 
      hiredate < (select hiredate from emp where ename ='SCOTT'
);

5. 如何查询工资比scott高或者雇佣时间比scott早的员工姓名和编号

select empno,ename from emp
where sal>(select sal from emp where ename ='SCOTT')
        or 
      hiredate < (select hiredate from emp where ename ='SCOTT'
);

多行子查询

6. 查询工资比所有的 SALESMAN 都高的员工编号，名字，工资

select empno,ename,sal from emp
where sal>ALL(select sal from emp where job ='SALESMAN');

7. 查询部门20中职务同部门10的员工一样的员工信息

select * from emp where deptno = 20
and job = ANY(select job from emp where deptno=10);

8. 查询员工中有哪些是领导

select * from emp where empno in
(select distinct mgr from emp where mgr is not null);

9. 查询每个部门平均薪水的等级

select t.deptno,t.avg_sal,s.grade from 
(select deptno,avg(sal) avg_sal from emp group by deptno) T --把查询的结果看作是一个表
,salgrade s where t.avg_sal between s.losal and s.hisal;
--将子查询作为一张表，实现多表连接查询