[英]Detecting circular references in SQL
我有下表:
CREATE TABLE X (
A SOMETYPE NOT NULL,
B SOMETYPE NOT NULL,
C SOMETYPE NULL,
PRIMARY KEY (A,B),
FOREIGN KEY (A,C) REFERENCES X (A,B)
);
存儲在X
中的實體按層次結構組織:如果存在行(A1,B1,C1)
且C1 IS NOT NULL
那么無論C2
是什么(A1,C1,C2)
它都被認為是(A1,C1,C2)
的“子”。 由於項目不能自行下降,我想將循環分層序列存在為非法:
-- legal
INSERT INTO X (A1,B1,NULL);
INSERT INTO X (A1,B2,B1);
INSERT INTO X (A1,B3,B2);
INSERT INTO X (A1,B4,B2);
-- currently legal, but I want to make it illegal
UPDATE X SET C = B1 WHERE B = B1; /* B1-B1 */
UPDATE X SET C = B2 WHERE B = B1; /* B1-B2-B1 */
UPDATE X SET C = B3 WHERE B = B1; /* B1-B2-B3-B1 */
UPDATE X SET C = B4 WHERE B = B1; /* B1-B2-B4-B1 */
UPDATE X SET C = B2 WHERE B = B2; /* B2-B2 */
UPDATE X SET C = B3 WHERE B = B2; /* B2-B3-B2 */
UPDATE X SET C = B4 WHERE B = B2; /* B2-B4-B2 */
UPDATE X SET C = B3 WHERE B = B3; /* B3-B3 */
UPDATE X SET C = B4 WHERE B = B4; /* B4-B4 */
我該怎么做呢?
或者,我可以在表中添加一個表示層次結構中“級別”的字段:
CREATE TABLE X (
A SOMETYPE NOT NULL,
B SOMETYPE NOT NULL,
C SOMETYPE NULL,
LEVEL INT NOT NULL,
PRIMARY KEY (A,B),
FOREIGN KEY (A,C) REFERENCES X (A,B)
);
然后我想要求當C IS NULL
時LEVEL
為0
,否則parent's LEVEL + 1
。
我正在使用SQL Server 2008 R2。
要檢查循環引用,我使用了觸發器和遞歸CTE:
CREATE TRIGGER trgIU_X_CheckCircularReferences
ON dbo.X
AFTER INSERT, UPDATE
AS
BEGIN
SET NOCOUNT ON;
DECLARE @Results TABLE ([Exists] BIT);
WITH CteHierarchy
AS
(
SELECT x.A, x.B, X.C, 1 AS [Type]
FROM inserted i
JOIN X x ON i.A = x.A AND i.C = x.B
UNION ALL
SELECT x.A, x.B, X.C, 2 AS [Type]
FROM CteHierarchy i
JOIN X x ON i.A = x.A AND i.C = x.B
WHERE NOT EXISTS
(
SELECT *
FROM inserted a
WHERE a.A = x.A AND a.B = x.B
)
)
INSERT @Results ([Exists])
SELECT TOP(1) 1
FROM CteHierarchy h
JOIN X x ON h.A = x.A AND h.C = x.B
OPTION(MAXRECURSION 1000);
IF EXISTS(SELECT * FROM @Results)
BEGIN
ROLLBACK;
RAISERROR('Circular references detected', 16, 1);
END
END
GO
現在,我們可以運行一些測試:
--Test 1 - OK
PRINT '*****Test 1 - OK*****';
SELECT * FROM X;
BEGIN TRANSACTION;
UPDATE X
SET C = 'B1'
WHERE B = 'B4';
SELECT * FROM X;
--This transaction can be commited without problems
--but I will cancel all modification so we can run the second test
ROLLBACK TRANSACTION;
PRINT '*****End of test 1*****';
GO
--Test 2 - NOT OK
PRINT '*****Test 2 - NOT OK*****';
SELECT * FROM X;
BEGIN TRANSACTION;
UPDATE X
SET C = 'B1'
WHERE B = 'B1';
--Useless in this case (test 2 & test 3)
--Read section [If a ROLLBACK TRANSACTION is issued in a trigger] from http://msdn.microsoft.com/en-us/library/ms181299.aspx
SELECT * FROM X;
--Useless
ROLLBACK TRANSACTION;
--Useless
PRINT '*****End of test 2*****';
GO
PRINT '*****Test 3 - NOT OK*****';
SELECT * FROM X;
BEGIN TRANSACTION;
UPDATE X
SET C = 'B4'
WHERE B = 'B1';
GO
結果:
*****Test 1 - OK*****
(4 row(s) affected)
(0 row(s) affected)
(1 row(s) affected)
(4 row(s) affected)
*****End of test 1*****
*****Test 2 - NOT OK*****
(4 row(s) affected)
(1 row(s) affected)
Msg 50000, Level 16, State 1, Procedure trgIU_X_CheckCircularReferences, Line 34
Circular references detected
Msg 3609, Level 16, State 1, Line 8
The transaction ended in the trigger. The batch has been aborted.
*****Test 3 - NOT OK*****
(4 row(s) affected)
(1 row(s) affected)
Msg 50000, Level 16, State 1, Procedure trgIU_X_CheckCircularReferences, Line 34
Circular references detected
Msg 3609, Level 16, State 1, Line 7
The transaction ended in the trigger. The batch has been aborted.
對於第二個測試,您可以看到此觸發器如何取消( ROLLBACK TRANSACTION
)事務,並且在UPDATE之后,沒有執行任何操作(在當前批處理中 )。
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