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oracle 面试题下 (七) rownum的运用

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Release： 2020-08-01 17:28:16

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oracle 面试题下 (七) rownum的运用

1 求经理人平均薪水最低的部门名称: select dname from dept where deptno = ( select deptno from ( select deptno,avg(sal)avg_sal from ( select e1.sal, e1.deptno,e2.mgr from emp e1 join emp e2 on (e1.empno=e2.mgr) ) group by deptno ) where avg_

1 求经理人平均薪水最低的部门名称:

select dname from dept where deptno =
(
select deptno from
(
select deptno,avg(sal)avg_sal from
(
select e1.sal, e1.deptno,e2.mgr from emp e1 join emp e2 on (e1.empno=e2.mgr)
) group by deptno
) where avg_sal =
(
select min(avg_sal) from
(
select deptno,avg(sal)avg_sal from
(
select e1.sal, e1.deptno,e2.mgr from emp e1 join emp e2 on (e1.empno=e2.mgr)
) group by deptno
)
)
)

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2 比普通员工的最高薪水还高的经理人名称

select ename from emp
where empno in (select distinct mgr from emp where mgr is not null) and sal >
(
select max(sal) from emp where empno not in
(select distinct mgr from emp where mgr is not null)
)

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其中(select distinct mgr from emp where mgr is not null) 注意空值的处理.

3 求薪水最高的前五雇员

* 关于隐藏属性 rownum :

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rownum 从1开始,每行递增 . 如查询表中的前五人:

select ename from emp where rownum < = 5;

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注意oracle 中 rownum 不可直接与> , = 连用, 如:select ename from emp where rownum >10;

当需要查找r>10时必须使用子查询:

select ename from (select ename ,rownum r from emp)where r >10;

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以上.

求薪水最高的前五人:

select ename ,sal from
( select ename ,sal from emp order by sal desc )
where rownum<=5;

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4 求薪水最高的第六个人到第十个人

select ename,sal from
(
select ename ,sal ,rownum r from
(
select ename ,sal from emp order by sal desc
)
)where r>=6 and r <=10

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为什么不能 select ename ,sal from

(select ename,sal ,rownum r from emp order by sal desc)

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where r >=6 and r <=10 这样写呢?

因为 (select ename,sal ,rownum r from emp order by sal desc) 这句话先取了emp的 rownum 然后再按sal 倒排, 无法达到取 sal 6到10名的效果.

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