COUNT rows from multiple joined tables - MSSQL

Question

I'm trying to make a simple forum for a school-project. I'm having some troubles with my query. I have 3 tables, that I'm trying to join. "Categories", "Threads" and "Post".

ERD

The table data

Category

Threads

Posts

(Even though the name-spelling is a little bit different in the DB, the columns and tables are the same, as shown in the image.) As soon as I join the second table ("Posts"), the result from the "Threads"-count is wrong. If I only left join and count the rows in "Threads", then the result is right.

When i do this:

    SELECT Category.Name AS CategoryName, Category.Description AS
    CategoryDescription, COUNT(Threads.Id) AS NumberOfThreads
    FROM Category
    LEFT JOIN Threads ON Threads.FkCategoryId = Category.Id GROUP
    BY Category.Name, Category.Description

The Result is

Which returns the actual number of rows in "Threads". All is good so far, BUT when i add the second join

    SELECT Category.Name AS CategoryName, Category.Description AS CategoryDescription,
    COUNT(Threads.Id) AS NumberOfThreads,
    COUNT(Posts.Id) AS NumberOfPosts
    FROM Category
    LEFT JOIN Threads ON Threads.FkCategoryId = Category.Id
    LEFT JOIN Posts ON Posts.FkThreadId = Threads.Id
    GROUP BY Category.Name, Category.Description

Then i get this result:

Which shows the actual number of rows in "Posts" as both "Threads" and "Posts".

I have also tried to combine multiple queries in one like this:

    SELECT Category.Name AS CategoryName, Category.[Description] AS
    CategoryDescription, COUNT(Threads.Id) AS NumberOfThreads
    FROM Category
    LEFT JOIN Threads ON Category.Id = Threads.FkCategoryId
    GROUP BY Category.Name, Category.Description;

    SELECT COUNT(Posts.Id) AS NumberOfPosts
    FROM Category
    LEFT JOIN Threads ON Category.Id = Threads.FkCategoryId
    LEFT JOIN Posts ON Threads.Id = Posts.FkThreadId
    GROUP BY Category.Id;

Which gives me this result:

which is the correct data. I would just rather combine it in 1 query, because I can't "Eval" "NumberOfPosts" form the second query.

I get this Error:

"DataBinding: 'System.Data.DataRowView' does not contain a property with the name 'NumberOfPosts'"

I have read and tried a lot of different things, and don't know how to solve it. I hope someone can help.

Answer 1

Please try something like the following:

SELECT CatThread.CategoryName, CatThread.CategoryDescription,
    CatThread.NumberOfThreads,
    COUNT(Posts.Id) AS NumberOfPosts
FROM 
   ( select Category.Id, Category.Name AS CategoryName, Category.Description AS CategoryDescription,
    COUNT(Threads.Id) AS NumberOfThreads
    from Category
      LEFT JOIN Threads ON Threads.FkCategoryId = Category.Id
    group by  Category.Id, Category.Name, Category.Description) CatThread
    LEFT JOIN Threads ON Threads.FkCategoryId = CatThread.Id
    LEFT JOIN Posts ON Posts.FkThreadId = Threads.Id
GROUP BY CatThread.CategoryName, CatThread.CategoryDescription,
    CatThread.NumberOfThreads

I did not create the tables and such to make sure this works, but I believe this query should give you what you want. You might have to tweak the syntax and such. The basic idea is to use a sub-query to get the Threads per Category row counts in a single row, which then can be joined to the Posts table.

Answer 2

I think you just missed join condition Post.FkUserId = Threads.FkUserId on the second join.

Try:

 LEFT JOIN Posts ON Threads.Id = Posts.FkThreadId  AND PostFkUserId = Threads.FkUserId

Basically you query matches more records because you join only on the Threads.Id = Posts.FkThreadId. In the Post table there is 7 records with FkThreadId that is 7,6,6,5,2,2,2.