部门：dept(deptno,dname)

员工：emp(ename,sal,deptno)

薪资等级：salgrade(grade,losal,hisal)

 平均工资的最小值及部门编号和工资等级及部门名称：

select t1.deptno,t1.avg_sal,grade,d.dname from (  select deptno,avg_sal,grade from (select deptno,avg(sal) avg_sal from emp group by deptno) t join salgrade s on (t.avg_sal between s.losal and s.hisal)  ) t1  join dept d on (t1.deptno = d.deptno)   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 s on (t.avg_sal between s.losal and s.hisal)  )  );