-- 练习
-- 1，返回拥有员工的部门名，部门号
select distinct dname,dept.deptno from dept join emp on dept.deptno = emp.deptno;

-- 2,工资水平多于smith的员工信息
select * from emp where sal > (select sal from emp where ename = 'smith');

-- 3,返回员工和所属经理的名字
select a.ename,b.ename from emp a  join emp b on a.mgr = b.empno;

select a.ename,b.ename from emp a,emp b where a.mgr = b.empno; 

-- 4,返回雇员的雇佣日期早于其经理雇佣日期的员工及其经理姓名
select a.ename,a.hiredate,b.hiredate,b.ename from emp a  join emp b on a.mgr = b.empno and a.hiredate<b.hiredate;

先写到这里，太困了，明天补完