Oracle “CONNECT BY” 使用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：

Oracle “CONNECT BY” 使用

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：

[ 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

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

初始化一些数据

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

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

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 DEPSTART WITH UPPERDEPID IS NULLCONNECT 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

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

    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