方法一：子查询法
SELECT a.ename,(SELECT ename from emp b where b.empno = a.mgr) as boss from emp a;
方法二：左连接法
SELECT a.ename , b.ename as boss_name from emp a left join emp b on b.empno = a.mgr;
方法三：自连接法
SELECT a.ename , b.ename from emp a , emp b where b.empno = a.mgr
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