Oracle支持层级结构的循环查询。我们可以将树形结构通过记录父子关系存储在关系表中。查询时使用connect by 语句可以选出某节点的所有子节点。

但是有时我们在项目中会遇到这种情况，即我们不需要取出所有的子节点，也许只需要取出它的下两层，或者三层。对于一层的情况，普通查询即可完成。

Oracle提供了level伪列来完成此项任务。

select user_id,level
from user
where level <= 2
start with user_id = 1278
connect by prior user_id = user_drtunder
order by level

这样就可以只查出两层的结构了

SQL> select empno,ename,mgr,level from emp start with mgr=7698 connect by prior mgr=empno;

     EMPNO ENAME                       MGR      LEVEL
---------- -------------------- ---------- ----------
      7499 ALLEN                      7698          1
      7698 BLAKE                      7839          2
      7839 KING                                     3
      7521 WARD                       7698          1
      7698 BLAKE                      7839          2
      7839 KING                                     3
      7654 MARTIN                     7698          1
      7698 BLAKE                      7839          2
      7839 KING                                     3
      7844 TURNER                     7698          1
      7698 BLAKE                      7839          2

     EMPNO ENAME                       MGR      LEVEL
---------- -------------------- ---------- ----------
      7839 KING                                     3
      7900 JAMES                      7698          1
      7698 BLAKE                      7839          2
      7839 KING                                     3

已选择15行。

SQL> select empno,ename,mgr,level from emp where level<=2 start with mgr=7698 connect by prior mgr=empno;

     EMPNO ENAME                       MGR      LEVEL
---------- -------------------- ---------- ----------
      7499 ALLEN                      7698          1
      7698 BLAKE                      7839          2
      7521 WARD                       7698          1
      7698 BLAKE                      7839          2
      7654 MARTIN                     7698          1
      7698 BLAKE                      7839          2
      7844 TURNER                     7698          1
      7698 BLAKE                      7839          2
      7900 JAMES                      7698          1
      7698 BLAKE                      7839          2

已选择10行。

SQL>

SQL> select empno,ename,mgr,level from emp  start with empno=7900 connect by prior mgr=empno;

     EMPNO ENAME                       MGR      LEVEL
---------- -------------------- ---------- ----------
      7900 JAMES                      7698          1
      7698 BLAKE                      7839          2
      7839 KING                                     3

SQL> select empno,ename,mgr,level from emp where level<=2 start with empno=7900 connect by prior mgr=empno;

     EMPNO ENAME                       MGR      LEVEL
---------- -------------------- ---------- ----------
      7900 JAMES                      7698          1
      7698 BLAKE                      7839          2