Question

I’m creating a forum, so I have created a table with posts. One of the fields is a Body with of the type XML. Now I would like to create a query that returns all the posts and the number of children of every post. I’m doing this with an aggregate function. I need to use a group by when I’m using aggregate function. When I use the field in the group by, I’ll get the following exception:

The XML data type cannot be compared or sorted, except when using the
IS NULL operator.

How can I solve this?

My query is:

SELECT 
    Post.PostId, Post.[Body], Count(Children.PostId)
FROM  
    dbo.Post Post, 
    dbo.Post Children 
WHERE
    Children.ParentId = Post.PostId
GROUP BY
    Post.PostId, 
    Post.[Body]

Answer 1

You can do the aggregation in a CTE then join onto that

WITH Children(Cnt, ParentId)
     AS (SELECT COUNT(*),
                ParentId
         FROM   dbo.Post
         GROUP  BY ParentId)
SELECT P.PostId,
       P.[Body],
       ISNULL(Cnt, 0) AS Cnt
FROM   dbo.Post P
       LEFT JOIN Children /*To include childless posts*/
         ON Children.ParentId = P.PostId
ORDER  BY P.PostId