Recursive query with CTE - SUM of child columns for a given parent

问题

I have a forum database that stores forum information in a single column. The forum allows for unlimited subforums.

Table name - forums

| ForumID | ParentForumID | Name | Description | TopicCount | ReplyCount | LastPost |

Given a ForumID as a parameter I am trying to SUM the TopicCount and ReplyCount for all child entries. I am also trying to return the latest LastPost, which is specified as DATETIME.

I've searched google and this forum and understand I should be using a recursive CTE but am having some difficulty understanding the syntax. Here is my CTE - work in progress.

   WITH CTE (ForumID, ParentForumID)
   AS
   (
       SELECT ForumID AS Descendant, ParentForumID as Ancestor
       FROM forums
       UNION ALL
       SELECT e.Ancestor
       FROM
          CTE as e
          INNER JOIN CTE AS d
          ON Descendant = d.ParentForumID
   )
   SELECT e.Descendant, SUM(TopicCount) AS topics, SUM(ReplyCount) AS replys
   FROM CTE e
   WHERE e.Ancestor = 1

Where 1 = Parameter for the forum ID.

Thanks in advance for the help!

回答1:

You're doing OK - you're quite close :-)

Basically, you need to:

define the initial forum to be picked before the CTE
create an "anchor" query to that forum defined
then iterate over all children and sum up the TopicCount and ReplyCount counters

So your code should look something like this:

DECLARE @RootForumID INT
SET @RootForumID = 1  -- or whatever you want...

;WITH CTE AS
(
   -- define the "anchor" query - select the chosen forum
   SELECT 
       ForumID, TopicCount, ReplyCount, LastPost
   FROM 
       dbo.forums
   WHERE
       ForumID = @RootForumID

   UNION ALL

   -- select the child rows
   SELECT 
       f.ForumID, f.TopicCount, f.ReplyCount, f.LastPost
   FROM 
       dbo.forums f
   INNER JOIN
       CTE on f.ParentForumID = CTE.ForumID
)
SELECT 
    SUM(TopicCount) AS topics, 
    SUM(ReplyCount) AS replys,
    MAX(LastPost) AS 'Latest Post' 
FROM 
    CTE

Of course, you could wrap this into a stored procedure that would take the initial "root" ForumID as a parameter .

来源：https://stackoverflow.com/questions/8837347/recursive-query-with-cte-sum-of-child-columns-for-a-given-parent

标签

sql-server

sql-server-2005

tsql