Oracle “CONNECT BY”是層次查詢子句，一般用於樹狀或者層次結果集的查詢。其語法是：
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[
START WITHcondition
]
CONNECTBY[
NOCYCLE ] condition
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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：告訴系統以哪個節點作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情況時，會返回ORA-01436錯誤。此時，需要在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必須要跟關鍵字NOCYCLE結合起來使用
接下來，用一些示例來說明“CONNECT BY”的用法。
[例1]
創建一個部門表，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
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--
Create table
createtableDEP
(
DEPID
number(10) notnull,
DEPNAME
varchar2(256),
UPPERDEPID
number(10)
)
;
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初始化一些數據
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SQL>
INSERTINTODEP(DEPID,
DEPNAME, UPPERDEPID) VALUES(0,
'總經辦',
null);
1
row inserted
SQL>
INSERTINTODEP(DEPID,
DEPNAME, UPPERDEPID) VALUES(1,
'開發部',
0);
1
row inserted
SQL>
INSERTINTODEP(DEPID,
DEPNAME, UPPERDEPID) VALUES(2,
'測試部',
0);
1
row inserted
SQL>
INSERTINTODEP(DEPID,
DEPNAME, UPPERDEPID) VALUES(3,
'Sever開發部',
1);
1
row inserted
SQL>
INSERTINTODEP(DEPID,
DEPNAME, UPPERDEPID) VALUES(4,
'Client開發部',
1);
1
row inserted
SQL>
INSERTINTODEP(DEPID,
DEPNAME, UPPERDEPID) VALUES(5,
'TA測試部',
2);
1
row inserted
SQL>
INSERTINTODEP(DEPID,
DEPNAME, UPPERDEPID) VALUES(6,
'項目測試部',
2);
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row inserted
SQL>
commit;
Commitcomplete
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SQL>
SELECT*
FROMDEP;
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
rowsselected
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現在我要根據“CONNECT BY”來實現樹狀查詢結果
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SQL>
SELECTRPAD(
'
',
2*(LEVEL-1),
'-')
|| DEPNAME "DEPNAME",
CONNECT_BY_ROOT
DEPNAME "ROOT",
CONNECT_BY_ISLEAF
"ISLEAF",
LEVEL,
SYS_CONNECT_BY_PATH(DEPNAME,
'/')
"PATH"
FROMDEP
START
WITHUPPERDEPID
ISNULL
CONNECTBYPRIORDEPID
= UPPERDEPID;
DEPNAME
ROOT ISLEAF LEVELPATH
------------------------------
------------------- ---------- ---------- --------------------------------------------------------------------------------
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
rowsselected
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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，如果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]
通過CONNECT BY生成序列
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SQL>
SELECTROWNUM
FROMDUAL
CONNECTBYROWNUM
<= 10;
ROWNUM
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rowsselected
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[例3]
通過CONNECT BY用於十六進度轉換爲十進制
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CREATEORREPLACEFUNCTIONf_hex_to_dec(p_str
INVARCHAR2)
RETURNVARCHAR2
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
SELECTSUM(DATA)
INTOv_return
FROM(SELECT(CASEupper(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'
ELSEsubstr(p_str,
rownum, 1)
END)
* power(16, length(p_str) - rownum) DATA
FROMdual
CONNECTBYrownum
<= length(p_str));
RETURNv_return;
EXCEPTION
WHENOTHERS
THEN
RETURNNULL;
END;
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說明：
1. CONNECT BY rownum <= length(p_str))對輸入的字符串進行逐個遍歷
2. 通過CASE語句，來解析十六進制中的A-F對應的10進制值