Connect scott/tiger@orcl;
select * from emp
select * from salgrade
select * from dept

1、查询Scott的工资等级
select grade from salgrade where losal<=(select sal from emp where ename=\'SCOTT\') and hisal >=(select sal from emp where ename=\'SCOTT\')

select grade from emp inner join salgrade on sal>=losal and sal<=hisal where ename=\'SCOTT\'

2、查询各部门的平均工资
select deptno,avg(sal) from emp group by deptno

3、查询平均工资最高的部门名称
技巧（只取第一条记录）：
select * from (select ename from emp order by ename) t1 where rownum <=1;

select dname from dept where deptno=(select * from (select deptno from emp group by deptno order by avg(sal) desc) t1 where rownum <=1);

4、查询管理Scott的直接管理者
select ename from emp where empno=(select mgr from emp where ename=\'SCOTT\')

select t2.ename from emp t1 inner join emp t2 on t1.mgr=t2.empno where t1.ename=\'SCOTT\'

5、查询薪金比“SMITH”多的所有员工（利用自连接表达）
select e2.ename from emp e1 inner join emp e2 on e1.sal<e2.sal where e1.ename=\'SMITH\'

select ename from emp where sal>(select sal from emp where ename=\'SMITH\')