Oracle “CONNECT BY” 使用

Oracle “CONNECT BY”是层次查询子句，通常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：告诉系统以哪一个节点做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状况时，会返回ORA-01436错误。此时，须要在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必需要跟关键字NOCYCLE结合起来使用


接下来，用一些示例来讲明“CONNECT BY”的用法

[例1]
建立一个部门表，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

-- DROP TALBE
DROP TABLE dep CASCADE CONSTRAINT;
COMMIT;
-- CREATE TABLE
CREATE TABLE dep
( depid      NUMBER(10) NOT NULL,
  depname    VARCHAR2(256),
  upperdepid NUMBER(10)
  );
COMMIT;

初始化一些数据

INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (0, 'General Deparment', null);
INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (1, 'Development', 0);
INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (2, 'QA', 0);
INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (3, 'Server Development', 1);
INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (4, 'Client Development', 1);
INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (5, 'TA', 2);
INSERT INTO DEP(DEPID, DEPNAME, UPPERDEPID) VALUES (6, 'Porject QA', 2);
COMMIT;

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

如今我要根据“CONNECT BY”来实现树状查询结果

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

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，若是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]
经过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
 
已选择10行。

[例3]
经过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. 经过CASE语句，来解析十六进制中的A-F对应的10进制值