查询数据的练习

1、取得每个薪水等级有多少员工

select
    a.grade,count(a.grade)
from
    (select s.grade,sal from emp e join salgrade s on e.sal between s.losal and s.hisal) a
group by
    a.grade

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2、列出所有员工及其领导的姓名

select
    e1.ename,e2.ename
from
    emp e1
join
    emp e2
on
    e1.mgr=e2.empno

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3、列出至少有 5 个员工的所有部门编号

select
    dname,count(dname)
from
    (select d.dname,e.deptno from emp e join dept d on e.deptno=d.deptno) z
group by
    dname
having
    count(dname)>=5

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