TSQL CTE: How to avoid circular traversal?

Question

I have written a very simple CTE expression that retrieves a list of all groups of which a user is a member.

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.

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
            )

I can't find an easy solution to back-track those groups that I have already recorded.

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?

Is there a better way?

Answer 1

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;

Result:

Parent Child Sentinel
------ ----- --------
A      B     A
B      C     A|B
C      D     A|B|C
D      A     A|B|C|D