自连接:最大的特点是:一猴子那个表看做两张表,自己连自己。
案例:找出每个员工的上级领导,要求显示员工名和对应的领导名。
下面是原始数据(可以是员工表也可以是领导表):
mysql> select empno,ename,mgr from emp; +-------+--------+------+ | empno | ename | mgr | +-------+--------+------+ | 7369 | SMITH | 7902 | | 7499 | ALLEN | 7698 | | 7521 | WARD | 7698 | | 7566 | JONES | 7839 | | 7654 | MARTIN | 7698 | | 7698 | BLAKE | 7839 | | 7782 | CLARK | 7839 | | 7788 | SCOTT | 7566 | | 7839 | KING | NULL | | 7844 | TURNER | 7698 | | 7876 | ADAMS | 7788 | | 7900 | JAMES | 7698 | | 7902 | FORD | 7566 | | 7934 | MILLER | 7782 | +-------+--------+------+
员工的领导编号 = 领导的员工编号
mysql> select e.ename as '员工名',b.ename'领导名' from emp e inner join emp b on e.mgr = b.empno; +--------+--------+ | 员工名 | 领导名 | +--------+--------+ | SMITH | FORD | | ALLEN | BLAKE | | WARD | BLAKE | | JONES | KING | | MARTIN | BLAKE | | BLAKE | KING | | CLARK | KING | | SCOTT | JONES | | TURNER | BLAKE | | ADAMS | SCOTT | | JAMES | BLAKE | | FORD | JONES | | MILLER | CLARK | +--------+--------+