TSQL CTE:如何避免循环遍历? [英] TSQL CTE: How to avoid circular traversal?
问题描述
我写了一个非常简单的CTE表达式,该表达式检索用户是其成员的所有组的列表。
I have written a very simple CTE expression that retrieves a list of all groups of which a user is a member.
规则是这样的,用户可以在多个组中,并且组可以嵌套,以便一个组可以是另一个组的成员,此外,组可以是另一个成员的共同成员,所以A组是B组的成员,B组也是A组的成员。
The rules goes like this, a user can be in multiple groups, and groups can be nested so that a group can be a member of another group, and furthermore, groups can be mutual member of another, so Group A is a member of Group B and Group B is also a member of Group A.
我的CTE显然是这样的产生无限递归:
My CTE goes like this and obviously it yields infinite recursion:
;WITH GetMembershipInfo(entityId) AS( -- entity can be a user or group
SELECT k.ID as entityId FROM entities k WHERE k.id = @userId
UNION ALL
SELECT k.id FROM entities k
JOIN Xrelationships kc on kc.entityId = k.entityId
JOIN GetMembershipInfo m on m.entityId = kc.ChildID
)
找到一个简单的解决方案来回溯我已经记录的那些组。
I can't find an easy solution to back-track those groups that I have already recorded.
我当时想在CTE中使用一个额外的varchar参数来记录我访问过的所有组的列表,但是使用varchar太粗糙了,不是
I was thinking of using an additional varchar parameter in the CTE to record a list of all groups that I have visited, but using varchar is just too crude, isn't it?
有更好的方法吗?
推荐答案
您需要在您的递归中积累一个前哨字符串。在下面的示例中,我有一个从A,B,C,D到然后回到A的循环关系,并且避免了前哨字符串的循环:
You need to accumulate a sentinel string within your recursion. In the following example I have a circular relationship from A,B,C,D, and then back to A, and I avoid a loop with the sentinel string:
DECLARE @MyTable TABLE(Parent CHAR(1), Child CHAR(1));
INSERT @MyTable VALUES('A', 'B');
INSERT @MyTable VALUES('B', 'C');
INSERT @MyTable VALUES('C', 'D');
INSERT @MyTable VALUES('D', 'A');
; WITH CTE (Parent, Child, Sentinel) AS (
SELECT Parent, Child, Sentinel = CAST(Parent AS VARCHAR(MAX))
FROM @MyTable
WHERE Parent = 'A'
UNION ALL
SELECT CTE.Child, t.Child, Sentinel + '|' + CTE.Child
FROM CTE
JOIN @MyTable t ON t.Parent = CTE.Child
WHERE CHARINDEX(CTE.Child,Sentinel)=0
)
SELECT * FROM CTE;
结果:
Parent Child Sentinel
------ ----- --------
A B A
B C A|B
C D A|B|C
D A A|B|C|D
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