ID  param
1   '1'
1   '2'
2  '1'
3  '2'

ID  param
1   '1/2'
2  '1'
3  '2'

SELECT id
, CAST(CASE SUM(/* DISTINCT*/ CAST(param AS INT)) 
       WHEN 1 THEN '1'
       WHEN 2 THEN '2'
       WHEN 3 THEN '1/2'
       END 
       AS CHAR(3)
      )param
FROM MyQuery
GROUP BY id

WITH S AS 
(SELECT * FROM (VALUES(1, '1'), (1, '2'), (2, '1'), (3, '2')) AS T(ID, param))
SELECT DISTINCT ID, LEFT(P.C, LEN(P.C) - 1) FROM S CROSS APPLY (SELECT T.param + '/' AS 'text()'
		  FROM S AS T WHERE T.ID = S.ID
		  FOR XML PATH ('')) AS P(C)