--查询平均薪水最高部门的部门编号
select deptno,avg_sal from
(select deptno,avg(sal) avg_sal from emp group by deptno)--当做一张表，否则没有表可以from
where avg_sal =
(select max(avg_sal)from (select deptno,avg(sal) avg_sal from emp group by deptno ))--当做一个值;
--查询平均薪水最高部门的部门名称
select dname from dept where deptno=
(select deptno from --当做一个值
(select deptno,avg(sal) avg_sal from emp group by deptno)
where avg_sal =
(select max(avg(sal))from emp group by deptno ) );--组函数的嵌套，最多两层，就是这种两层（（））

--查询部门平均薪水的等级
select deptno ,avg_sal, grade from (select deptno,avg(sal) avg_sal from emp group by deptno) t
join salgrade on avg_sal between losal and hisal;

--查询平均薪水等级最低的部门的名称--利用视图可以简化程序代码
select dname from dept where deptno=
( select deptno from
((select deptno ,avg_sal, grade from (select deptno,avg(sal) avg_sal from emp group by deptno) t
join salgrade on avg_sal between losal and hisal))
where grade=
(select min(grade) from
(select deptno ,avg_sal, grade from (select deptno,avg(sal) avg_sal from emp group by deptno) t
join salgrade on avg_sal between losal and hisal)));

--利用视图简化代码
create view my_view as (select deptno ,avg_sal, grade from (select deptno,avg(sal) avg_sal from emp group by deptno) t
join salgrade on avg_sal between losal and hisal);
--创建视图要用sys用户来给scott用户授权
用sys用户登录成功后，
grant create table,create view to scott;
授权成功后才可以利用scott普通用户创建视图；
--用视图来简化上面的代码
select dname from dept where deptno=
( select deptno from
(my_view )
where grade=
(select min(grade) from
my_view ));
--求比普通员工最高薪水还要高的经理人的名称
普通员工：即不在mgr这个里面的就是普通员工
经理人：在mgr这个里面的就是经理人
--普通员工的最高薪水
select max(sal) from emp where empno not in(select distinct mgr from emp where mgr is not null);--对于有null值的要进行这样处理

--再求比这个最高工资还要高的经理人的名称
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));

--求部门经理人中平均薪水最低的部门名称
分析：
哪些部门中有哪些人是经理人，这些经理人所在部门，薪水是多少。
select dname from dept d join (
select deptno,avg(sal) avg_sal from (select ename,sal,deptno from emp where empno in(select mgr from emp )) group by deptno ) s
on d.deptno =s.deptno
where avg_sal=
(select min(avg_sal) from (select deptno,avg(sal) avg_sal from (select ename,sal,deptno from emp where empno in(select mgr from emp )) group by deptno));

--求薪水最高的前5名的雇员
select ename,sal from (select ename,sal from emp order by sal desc) where rownum <=5;

mysql的分页是limit a,b oracle的分页比较特殊，没有limit只有rownum，这个是不显示的一个伪字段。这个字段只能使用小于或者小于等于，不能使用大于或者大于等于。
如果分页只能先查询出rownum这列 select rownum from emp，然后从这里面再rownum>或者<哪个数字。
select rownum ,ename from (select rownum no from emp) t where no>3 and no<10;

--求薪水最高的第6名到第10名的雇员
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;

--求最后入职的5名员工(按照入职时间降序排列，取出前五个即可)

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

--比较效率
select * from emp where deptno=10 and ename like ‘%A%‘ --这个效率高些，跟java的短路与类似
select * from emp where ename like ‘%A% and deptno=10

oracle的sql语句训练