Note: it would be great if support was added for a STRING_AGG function. Here's how simple it is to write the same order-preserving query in Postgres SQL (as an example):

;WITH Blah(col1, i) as
(
          SELECT 'abc',1
    UNION SELECT 'def',2
    UNION SELECT 'ghi',3
)
SELECT STRING_AGG(col1, ',' ORDER BY i ASC)
FROM Blah