Return columns as comma separated, for more than one condition on key or more than one row
Solution 1:
SELECT t1.Key1, t1.Key2,
STUFF((SELECT ', ' + ColumnKey
FROM Table1 t2
WHERE t2.Key1 = t1.Key1
AND t2.Key2 = t1.Key2
ORDER BY ColumnKey
FOR XML PATH('') ),1,2,'') AS TColumnKey
FROM Table1 t1
GROUP BY t1.Key1, t1.Key2;