Oracle 「CONNECT BY」 使用

Oracle 「CONNECT BY」是層次查詢子句，通常用於樹狀或者層次結果集的查詢。

其語法是：

[ 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).

說明： 
1. START WITH：告訴系統以哪一個節點做爲根結點開始查找並構造結果集，該節點即爲返回記錄中的最高節點。 
2. 當分層查詢中存在上下層互爲父子節點的狀況時，會返回ORA-01436錯誤。此時，須要在connect by後面加上NOCYCLE關鍵字。同時，可用connect_by_iscycle僞列定位出存在互爲父子循環的具體節點。 connect_by_iscycle必需要跟關鍵字NOCYCLE結合起來使用


接下來，用一些示例來講明「CONNECT BY」的用法

[例1]
建立一個部門表，這個表有三個字段，分別對應部門ID，部門名稱，以及上級部門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;

初始化一些數據

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

如今我要根據「CONNECT BY」來實現樹狀查詢結果

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

說明： 
1. CONNECT_BY_ROOT 返回當前節點的最頂端節點 
2. CONNECT_BY_ISLEAF 判斷是否爲葉子節點，若是這個節點下面有子節點，則不爲葉子節點 
3. LEVEL 僞列表示節點深度 
4. SYS_CONNECT_BY_PATH函數顯示詳細路徑，並用「/」分隔

[例2]
經過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
 
已選擇10行。

[例3]
經過CONNECT BY用於十六進度轉換爲十進制

CREATE OR REPLACE FUNCTION f_hex_to_dec(p_str IN VARCHAR2) RETURN VARCHAR2 IS
----------------------------------------------------------------------------------------------------------------------
-- 對象名稱: f_hex_to_dec
-- 對象描述: 十六進制轉換十進制
-- 輸入參數: p_str 十六進制字符串
-- 返回結果: 十進制字符串
-- 測試用例: 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;

說明： 1. CONNECT BY rownum <= length(p_str))對輸入的字符串進行逐個遍歷 2. 經過CASE語句，來解析十六進制中的A-F對應的10進制值