Question

I’m building a forum. Now I would like to create a query that returns the number of reactions on each post. A reaction is defined as all the posts under a certain post. I would like to use CTE to achieve this.

So fare I’ve got this:

;WITH Reaction(Cnt, ParentId) AS 
(
    SELECT  COUNT(*), ParentId
    FROM   dbo.Post
    GROUP  BY ParentId
)

SELECT ISNULL(Cnt, 0), Post.*
FROM   dbo.Post Post
       LEFT JOIN Reaction
         ON Reaction.ParentId = Post.PostId 

This is listing all the ‘direct’ posts. Now I’ve got to make this query count the entire tree, but I’m stuck. I’ve been reading a bit on CTE and I know you can make recursive queries, but I don’t know how to solve the problem of creating a recursive query that counts.

Answer 1

• Build the post list first
• Aggregate later
• Don’t need LEFT JOIN in the recursive clause
• Need to get ParentID in the recursive clause for the next level, otherwise you just loop PostId to PostID at the same level

Something like this:

;WITH Reaction AS 
(
    SELECT ParentId
    FROM dbo.Post
    UNION ALL 
    SELECT P.ParentId
    FROM dbo.Post P JOIN Reaction R ON R.ParentId = P.PostId
)
SELECT COUNT(*), ParentId FROM Reaction GROUP BY ParentId;

Edit, updated with in-line count

;WITH Reaction AS
(
   SELECT ParentId FROM dbo.Post 
   UNION ALL 
   SELECT P.ParentId FROM dbo.Post P JOIN Reaction R ON R.ParentId = P.PostId
)
SELECT 
     COUNT(*) OVER (PARTITION BY Reactions.ParentId) AS Cnt,
     * 
FROM Post LEFT JOIN Reaction ON Reaction.ParentId = Post.PostId