【层次查询】Hierarchical Queries之CONNECT_BY_ISLEAF伪列

2020-02-12 11:22:47

有关层次查询之前的文章参考如下。
【层次查询】Hierarchical Queries之LEVEL应用
http://space.itpub.net/519536/viewspace-623916
【层次查询】Hierarchical Queries之“树的遍历”
http://space.itpub.net/519536/viewspace-623809
【层次查询】Hierarchical Queries之CONNECT_BY_ISCYCLE伪列
http://space.itpub.net/519536/viewspace-624032

继LEVEL和CONNECT_BY_ISCYCLE伪列之后，后一个可用的伪列名字叫做CONNECT_BY_ISLEAF，正如这个伪列名字中描述的那样，他可以指示出哪些是“叶子节点”，就是这么简单。
如果发现是叶子节点，该伪列会返回“1”，反之返回记录“0”。

1.回望那棵关系“树”，很直观，F、G和E节点是叶子节点。
      A
     / \
    B   C
   /   /
D   E
/ \
F   G

2.重温一下阐述上图的T表数据
sec@ora10g> select * from t;

X                   Y          Z
---------- ---------- ----------
A                   1
B                   2          1
C                   3          1
D                   4          2
E                   5          3
F                   6          4
G                   7          4

7 rows selected.

3.结合LEVEL伪列看一下CONNECT_BY_ISLEAF的直观效果
sec@ora10g> col tree for a16
sec@ora10g> select lpad(' ',2*(level-1))||x tree, CONNECT_BY_ISLEAF from t start with x = 'A' connect by NOCYCLE prior y=z;

TREE             CONNECT_BY_ISLEAF
---------------- -----------------
A                                0
B                              0
    D                            0
      F                          1
      G                          1
C                              0
    E                            1

7 rows selected.

是不是有一种豁然开朗的感觉，对头，CONNECT_BY_ISLEAF功能就是这么简单。

4.如何仅筛选出上面结果中所有叶子节点？
sec@ora10g> select lpad(' ',2*(level-1))||x tree, CONNECT_BY_ISLEAF from t where CONNECT_BY_ISLEAF=1 start with x = 'A' connect by NOCYCLE prior y=z;

TREE             CONNECT_BY_ISLEAF
---------------- -----------------
      F                          1
      G                          1
    E                            1

5.得到每一层级叶子节点
可以理解下面的SQL是对“树”的逐层消除的过程。
1）不做消除，因为我们的树只有4层。
（1）SQL
sec@ora10g> select lpad(' ',2*(level-1))||x tree, CONNECT_BY_ISLEAF, LEVEL from t where CONNECT_BY_ISLEAF=1 start with x = 'A' connect by NOCYCLE prior y=z and level<=4;

TREE             CONNECT_BY_ISLEAF      LEVEL
---------------- ----------------- ----------
      F                          1          4
      G                          1          4
    E                            1          3

（2）对应的“树”
      A
     / \
    B   C
   /   /
D   E
/ \
F   G

2）消除第四层
（1）SQL
sec@ora10g> select lpad(' ',2*(level-1))||x tree, CONNECT_BY_ISLEAF, LEVEL from t where CONNECT_BY_ISLEAF=1 start with x = 'A' connect by NOCYCLE prior y=z and level<=3;

TREE             CONNECT_BY_ISLEAF      LEVEL
---------------- ----------------- ----------
    D                            1          3
    E                            1          3

（2）对应的“树”
      A
     / \
    B   C
   /   /
D   E

3）消除第三层
（1）SQL
sec@ora10g> select lpad(' ',2*(level-1))||x tree, CONNECT_BY_ISLEAF, LEVEL from t where CONNECT_BY_ISLEAF=1 start with x = 'A' connect by NOCYCLE prior y=z and level<=2;

TREE             CONNECT_BY_ISLEAF      LEVEL
---------------- ----------------- ----------
B                              1          2
C                              1          2

（2）对应的“树”
      A
     / \
    B   C

4）消除第二层
（1）SQL
sec@ora10g> select lpad(' ',2*(level-1))||x tree, CONNECT_BY_ISLEAF, LEVEL from t where CONNECT_BY_ISLEAF=1 start with x = 'A' connect by NOCYCLE prior y=z and level<=1;

TREE             CONNECT_BY_ISLEAF      LEVEL
---------------- ----------------- ----------
A                                1          1

（2）对应的“树”，此时只剩根节点了。
      A

6.小结
有关层次查询中可用的伪列有LEVEL、CONNECT_BY_ISCYCLE还是本文所述的CONNECT_BY_ISLEAF伪列，这些伪列都是从可用性角度提出来的。稍作组合便可以得到意想不到的效果。更多有趣的细节请大家慢慢体会。

今天是2009年的后一天，祝愿每一位好朋友新年快乐，吉祥如意，身体健康。

Good luck.

secooler
09.12.31

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