WITH t(idx, title) AS
(
SELECT 1 idx, 'aaa' title
UNION ALL SELECT 2, 'ddd'
UNION ALL SELECT 3, 'aaa'
UNION ALL SELECT 4, 'aaa'
)
SELECT title
     , STUFF((SELECT CONCAT(' | ', s.idx)
                FROM t s
               WHERE s.title = m.title
               ORDER BY s.idx
                 FOR XML PATH('')
              ), 1, 3, '') idxs
  FROM t m
 GROUP BY title
;
-- http://www.gurubee.net/article/55512