Question

I’m trying to test the first answer to this question:

SQL – message schema – need to find an existing message thread given a set of users

The first answer to this question is written in DB2 and I’m having a hard time converting the answer to TSQL. Can someone help me figure this out? Here’s the query:

WITH Selected_Users(id) as (VALUES (@id1), (@id2), --etc--),
     Threads(id) as (SELECT DISTINCT threadFk
                     FROM ThreadMembers as a
                     JOIN Selected_Users as b
                     ON b.id = a.userFk)
SELECT a.id
FROM Threads as a
WHERE NOT EXISTS (SELECT '1'
                  FROM ThreadMembers as b
                  LEFT JOIN Selected_Users as c
                  ON c.id = b.userFk
                  WHERE c.id IS NULL
                  AND b.threadFk = a.id)
AND NOT EXISTS (SELECT '1'
                FROM Selected_Users as b
                LEFT JOIN ThreadMembers as c
                ON c.userFk = b.id
                AND c.threadFk = a.id
                WHERE c.userFk IS NULL) 

The description of the query is part of the answer, which helps a lot. The first part of the query creates a temp table called Selected_Users, but I’m not sure how this would be done. Thanks in advance!

Answer 1

I don’t think T-SQL allows for the list syntax that DB2 does.
As Andriy M points out, SQL 2008+ does allow a pretty similar syntax:

WITH Selected_Users(id) AS (
     SELECT Id FROM (
        VALUES (@id1), (@id2), --etc--
     ) AS V(Id)
),
....

Or you could create a real temp table (or variable):

DECLARE @selected_Users TABLE (id int);
INSERT @selected_Users VALUES 
 (@id1),
 (@id2),
 --etc.--
 ; --make sure to close with semi-colon before WITH CTE

and then replace Selected_Users with @selected_Users in the rest of the query. Or change the initial CTE to:

WITH Selected_Users(id) AS (
    SELECT * FROM @selected_Users
),
....

Or, you could do a UNION ALL:

WITH Selected_Users(id) AS (
   SELECT @id1
   UNION ALL SELECT @id2 
   UNION ALL SELECT @id3
   --etc.--
),
....