MySQL中的樹形查詢

樹形查詢本質就是遞歸算法(自己調用自己)，Oracle很早就支持了樹形查詢

MySQL中可以使用 WITH RECURSIVE 來實現樹形查詢

比如在Oracle中scott賬戶下，找出員工編號爲7369所有的上級：

SQL> select empno, ename, job, mgr, level
  2    from emp
  3   start with empno = 7369
  4  connect by empno = prior mgr;
 
EMPNO ENAME      JOB         MGR      LEVEL
----- ---------- --------- ----- ----------
 7369 SMITH      CLERK      7902          1
 7902 FORD       ANALYST    7566          2
 7566 JONES      MANAGER    7839          3
 7839 KING       PRESIDENT                4

在MySQL8.0.19中：

mysql> WITH RECURSIVE x(empno,ename,job,mgr,lv) AS
    ->  (SELECT empno,ename,job,mgr,1 lv from emp where empno=7369
    ->  UNION ALL
    ->  select t1.empno,t1.ename,t1.job,t1.mgr,lv+1 from emp t1,x where t1.empno=x.mgr
    ->  )
    -> select * from x;
+-------+-------+-----------+------+------+
| empno | ename | job       | mgr  | lv   |
+-------+-------+-----------+------+------+
|  7369 | SMITH | CLERK     | 7902 |    1 |
|  7902 | FORD  | ANALYST   | 7566 |    2 |
|  7566 | JONES | MANAGER   | 7839 |    3 |
|  7839 | KING  | PRESIDENT | NULL |    4 |
+-------+-------+-----------+------+------+
4 rows in set (0.00 sec)