WITH Data AS (
SELECT 1 UserId, 'A' Code
UNION ALL
SELECT 1, 'C5'
UNION ALL
SELECT 1, 'X'
UNION ALL
SELECT 2, 'V3'
UNION ALL
SELECT 3, 'B'
UNION ALL
SELECT 3, 'D'
UNION ALL
SELECT 3, NULL
UNION ALL
SELECT 3, 'F4'
UNION ALL
SELECT 4, NULL
)
SELECT U.UserId, STUFF((
SELECT ','+Code FROM Data WHERE Data.UserID = U.UserID FOR XML PATH('')
), 1, 1, '') Code
FROM (SELECT DISTINCT UserID FROM Data) U
Basta sostituire i Data
CTE con il nome del tuo tavolo e il gioco è fatto.