Question

I have the following tables:

Categories
=================================
CategoryID |   ParentID   | Text 
---------------------------------
     1           NULL       Text
     2            1         Text
     3           NULL       Text
     4            1         Text

Items
=================================
  ItemID  |  CategoryID  | Text
---------------------------------
     1           1         Text
     2           2         Text
     3           4         Text
     4           3         Text

Bear in mind this is not an n-level hierarchy, the categories have only 2 levels, so no category can have a parentID of 2 for instance.

What I am looking for is a way to return the categories with an extra column which displays the number of items that the category owns (including its subcategories).

i.e. I’m looking for a single query (or procedure) which can return something like the following:

      Categories
============================================
CategoryID |   ParentID   | Text  |  Count
--------------------------------------------
     1           NULL       Text      3
     2            1         Text      1
     3           NULL       Text      1
     4            1         Text      1

My current method of getting the items associated to a category is as follows (Given a categoryID, @CategoryID):

SELECT * FROM Items
WHERE CategoryID 
IN (SELECT CategoryID FROM Categories where ParentID = @CategoryID or CategoryID = @CategoryID)

My problem is that I cannot seem to link this to a select query for the categories themselves.
It’s probably very simple, but I have tried methods using CTE’s, various group by clauses, but the hierarchical nature of the categories seems to throw my logic off.

Thanks for any help!

EDIT: The query also needs to account for categories with no items associated to them

Answer 1

You’d have to join items to categories, and group by on any fields you need from the category table:

SELECT  c.CategoryID
,       c.ParentID
,       c.Text
,       count(distinct i.ItemID)
FROM    Categories c
LEFT JOIN
        Categories c2
ON      c.CategoryID = c2.ParentID
LEFT JOIN    
        Items i
ON      i.CategoryID = c.CategoryID
        or i.CategoryID = c2.CategoryID
GROUP BY
        c.CategoryID
,       c.ParentID
,       c.Text