1.内连接，等值查询
查询员工对应的部门名称
select e.ename,d.dname
from emp as e
inner join dept as d
on e.deptno=d.deptno;
2.自连接：把一张表看成两张表使用
查询员工对应的领导名称，要求显示员工名称和领导名称
select a.ename,b.ename as leadername
from emp as a
inner join emp as b
on a.mgr=b.empno;
select e.ename,d.dname
from emp as e
inner join dept as d
on e.deptno=d.deptno;
select a.ename,b.ename as leadername
from emp as a
inner join emp as b
on a.mgr=b.empno;