Question

I have a query that works:

DECLARE @ProductID int
SET @ProductID = '1234'

SELECT DISTINCT TOP 12 a.ProductID
FROM A a
WHERE a.CategoryID IN (SELECT b.CategoryID FROM B b WHERE b.ProductID = @ProductID)
AND a.ProductID != @ProductID

It returns a list of 12 product numbers, all unique.

I need to store these results in a variable, comma separated, because that’s what 3rd party stored procedure needs. So I have this:

 DECLARE @ProductID int
 DECLARE @relatedprods varchar(8000)
 SET @ProductID = '1234'
 SET @relatedprods = ''

 SELECT TOP 12 @relatedprods = @relatedprods + CONVERT(VARCHAR(20), a.ProductID) + ', '
   FROM A a
   WHERE a.CategoryID IN (SELECT b.CategoryID FROM B b WHERE B.ProductID = @ProductID)
   AND a.ProductID != @ProductID

 SELECT @relatedprods

Now, none of these are distinct, but it is returning 12 rows.

Now I add the ‘distinct’ back in, like in the first query:

 DECLARE @ProductID int
 DECLARE @relatedprods varchar(8000)
 SET @ProductID = '1234'
 SET @relatedprods = ''

 SELECT DISTINCT TOP 12 @relatedprods = @relatedprods + CONVERT(VARCHAR(20), a.ProductID) + ', '
   FROM A a
   WHERE a.CategoryID IN (SELECT b.CategoryID FROM B b WHERE B.ProductID = @ProductID)
   AND a.ProductID != @ProductID

 SELECT @relatedprods

Only one product is returned in the comma separated list! Does ‘distinct’ not work in assignment statements? What did I do wrong? Or is there a way to get around this?

Thanks in advance!

CONCLUSION:

I have no idea what causes this problem, though the guess proposed seems logical. I was able to solve this problem via sub-query, and am posting it so others can see the solution:

 DECLARE @ProductID int
 DECLARE @relatedprods varchar(8000)
 SET @ProductID = '1234'
 SET @relatedprods = ''

 SELECT @relatedprods = @relatedprods + CONVERT(VARCHAR(20), c.ProductID) + ',' 
    FROM (SELECT DISTINCT TOP 12 a.ProductID FROM A a WHERE a.CategoryID IN 
      (SELECT b.CategoryID 
      FROM B b 
      WHERE B.ProductID = @ProductID) 
    AND a.ProductID != @ProductID ) c

 SET @relatedprods = SUBSTRING(@relatedprods, 0, LEN(@relatedprods))
 SELECT @relatedprods

Answer 1

Get the 12 records in a subquery:

declare
  @ProductID int,
  @relatedprods varchar(8000)

set @ProductID = '1234'
set @relatedprods = ''

select @relatedprods = @relatedprods + cast(ProductID as varchar) + ','
from (
  select distinct top 12 a.ProductId
  from A a
  inner join B b on b.CategoryID = a.CategoryID
  where  B.ProductID = @ProductID and a.ProductID != @ProductID
) x