connect by 用法

    Oracle “CONNECT BY”是层次查询子句，一般用于树状或者层次结果集的查询。其语法是：

1 2	[ START WITH condition ] CONNECT BY [ NOCYCLE ] condition

The start with .. connect by clause can be used to select data that has a hierarchical relationship (usually some sort of parent->child (boss->employee or thing->parts).

 

说明： 
1. START WITH：告诉系统以哪个节点作为根结点开始查找并构造结果集，该节点即为返回记录中的最高节点。 
2. 当分层查询中存在上下层互为父子节点的情况时，会返回ORA-01436错误。此时，需要在connect by后面加上NOCYCLE关键字。同时，可用connect_by_iscycle伪列定位出存在互为父子循环的具体节点。 connect_by_iscycle必须要跟关键字NOCYCLE结合起来使用

 

    接下来，用一些示例来说明“CONNECT BY”的用法。

 

[例1]

创建一个部门表，这个表有三个字段，分别对应部门ID，部门名称，以及上级部门ID

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-- Create table create table DEP (   DEPID      number(10) not null,   DEPNAME    varchar2(256),   UPPERDEPID number(10) ) ;

初始化一些数据

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SQL> INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (0, '总经办', null); 1 row inserted   SQL> INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (1, '开发部', 0); 1 row inserted   SQL> INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (2, '测试部', 0); 1 row inserted   SQL> INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (3, 'Sever开发部', 1); 1 row inserted   SQL> INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (4, 'Client开发部', 1); 1 row inserted   SQL> INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (5, 'TA测试部', 2); 1 row inserted   SQL> INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (6, '项目测试部', 2); 1 row inserted   SQL> commit; Commit complete

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SQL> SELECT * FROM DEP;         DEPID DEPNAME                                                                           UPPERDEPID ----------- -------------------------------------------------------------------------------- -----------           0 General Deparment                                                                          1 Development                                                                                0           2 QA                                                                                         0           3 Server Development                                                                         1           4 Client Development                                                                         1           5 TA                                                                                         2           6 Porject QA                                                                                 2   7 rows selected

现在我要根据“CONNECT BY”来实现树状查询结果

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SQL> SELECT RPAD( ' ', 2*(LEVEL-1), '-' ) || DEPNAME "DEPNAME", CONNECT_BY_ROOT DEPNAME "ROOT", CONNECT_BY_ISLEAF "ISLEAF", LEVEL , SYS_CONNECT_BY_PATH(DEPNAME, '/') "PATH" FROM DEP START WITH UPPERDEPID IS NULL CONNECT BY PRIOR DEPID = UPPERDEPID;   DEPNAME                        ROOT                    ISLEAF      LEVEL PATH ------------------------------ ------------------- ---------- ---------- -------------------------------------------------------------------------------- General Deparment              General Deparment            0          1 /General Deparment  -Development                  General Deparment            0          2 /General Deparment/Development  ---Server Development         General Deparment            1          3 /General Deparment/Development/Server Development  ---Client Development         General Deparment            1          3 /General Deparment/Development/Client Development  -QA                           General Deparment            0          2 /General Deparment/QA  ---TA                         General Deparment            1          3 /General Deparment/QA/TA  ---Porject QA                 General Deparment            1          3 /General Deparment/QA/Porject QA                                                      7 rows selected

说明： 
1. CONNECT_BY_ROOT 返回当前节点的最顶端节点 
2. CONNECT_BY_ISLEAF 判断是否为叶子节点，如果这个节点下面有子节点，则不为叶子节点 
3. LEVEL 伪列表示节点深度 
4. SYS_CONNECT_BY_PATH函数显示详细路径，并用“/”分隔

[例2]

通过CONNECT BY生成序列

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SQL> SELECT ROWNUM FROM DUAL CONNECT BY ROWNUM <= 10;       ROWNUM ----------          1          2          3          4          5          6          7          8          9         10   10 rows selected

[例3]

通过CONNECT BY用于十六进度转换为十进制

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CREATE OR REPLACE FUNCTION f_hex_to_dec(p_str IN VARCHAR2) RETURN VARCHAR2 IS     ----------------------------------------------------------------------------------------------------------------------     -- 对象名称: f_hex_to_dec     -- 对象描述: 十六进制转换十进制     -- 输入参数: p_str 十六进制字符串     -- 返回结果: 十进制字符串     -- 测试用例: SELECT f_hex_to_dec('78A') FROM dual;     ----------------------------------------------------------------------------------------------------------------------     v_return  VARCHAR2(4000);   BEGIN     SELECT SUM(DATA) INTO v_return       FROM (SELECT (CASE upper(substr(p_str, rownum, 1))                      WHEN 'A' THEN '10'                      WHEN 'B' THEN '11'                      WHEN 'C' THEN '12'                      WHEN 'D' THEN '13'                      WHEN 'E' THEN '14'                      WHEN 'F' THEN '15'                      ELSE substr(p_str, rownum, 1)                    END) * power(16, length(p_str) - rownum) DATA               FROM dual             CONNECT BY rownum <= length(p_str));     RETURN v_return;   EXCEPTION     WHEN OTHERS THEN       RETURN NULL;   END;

说明：

1. CONNECT BY rownum <= length(p_str))对输入的字符串进行逐个遍历

2. 通过CASE语句，来解析十六进制中的A-F对应的10进制值