SQL Server: How to limit CTE recursion to rows just recursivly added?

Question

Simpler Example

Let\'s try a simpler example, so people can wrap their heads around the concepts, and have a practical example that you can copy&paste into SQL

Answer 1

The issue is with the Sql Server default recursion limit (100). If you try your example at the top with the anchor restriction removed (also added Order By):

WITH NodeChildren AS
(
   --initialization
   SELECT ParentNodeID, NodeID, 1 AS GenerationsRemoved
   FROM Nodes

   UNION ALL

   --recursive execution
   SELECT P.ParentNodeID, N.NodeID, P.GenerationsRemoved + 1
   FROM NodeChildren AS P
      inner JOIN Nodes AS N
      ON P.NodeID = N.ParentNodeID
)
SELECT ParentNodeID, NodeID, GenerationsRemoved
FROM NodeChildren
ORDER BY ParentNodeID ASC

This produces the desired results. The problem you aare facing is with a larger number of rows you will recirse over 100 times which is a default limit. This can be changed by adding option (max recursion x) after your query, where x is a number between 1 and 32767. x can also be set to 0 which sets no limit however could very quickly have a very detrimental impact on your server performance. Clearly as the number of rows in Nodes increases, the number of recursions will can rise very quickly and I would avoid this approach unless there was a known upper limit on the rows in the table. For completeness, the final query should look like:

 WITH NodeChildren AS
    (
       --initialization
       SELECT ParentNodeID, NodeID, 1 AS GenerationsRemoved
       FROM Nodes

       UNION ALL

       --recursive execution
       SELECT P.ParentNodeID, N.NodeID, P.GenerationsRemoved + 1
       FROM NodeChildren AS P
          inner JOIN Nodes AS N
          ON P.NodeID = N.ParentNodeID
    )
    SELECT * 
    FROM NodeChildren
    ORDER BY ParentNodeID
    OPTION (MAXRECURSION 32767)

Where 32767 could be adjusted downwards to fit your scenario

Answer 2

Try this:

WITH Nodes AS
(
   --initialization
   SELECT ParentNodeID, NodeID, 1 AS GenerationsRemoved
   FROM ##Nodes

   UNION ALL

   ----recursive execution
   SELECT P.ParentNodeID, N.NodeID, P.GenerationsRemoved + 1
   FROM Nodes AS P
      INNER JOIN ##Nodes AS N
      ON P.NodeID = N.ParentNodeID
   WHERE P.GenerationsRemoved <= 10

)
SELECT ParentNodeID, NodeID, GenerationsRemoved
FROM Nodes
ORDER BY ParentNodeID, NodeID, GenerationsRemoved

Basically removing the "only show me absolute parents" from the initialization query; That way it generates the results starting from each of them and decending from there. I also added in the "WHERE P.GenerationsRemoved <= 10" as an infinite recursion catch(replace 10 with any number up to 100 to fit your needs). Then add the sort so it looks like the results you wanted.