Find the positive divisors!
PostgreSQL, 176 bytes
WITH c AS(SELECT * FROM(SELECT 6v)t,generate_series(1,sqrt(v)::int)s(r)WHERE v%r=0)
SELECT string_agg(r::text,',' ORDER BY r)
FROM(SELECT r FROM c UNION SELECT v/r FROM c)s

SqlFiddleDemo
Input:  (SELECT ...v)
How it works:

(SELECT ...v) - input
generate_series(1, sqrt(v)::int) - numbers from 1 to sqrt(n)
WHERE v%r=0 -filter divisors
wrap with common table expression to refer twice
SELECT r FROM c UNION SELECT v/r FROM c generete rest of divisors and combine
SELECT string_agg(r::text,',' ORDER BY r) produce final comma separated result


Input as table:
WITH c AS(SELECT * FROM i,generate_series(1,sqrt(v)::int)s(r)WHERE v%r=0)
SELECT v,string_agg(r::text,',' ORDER BY r)
FROM(SELECT v,r FROM c UNION SELECT v,v/r FROM c)s
GROUP BY v

SqlFiddleDemo
Output:
╔═════╦════════════════╗
║ v   ║   string_agg   ║
╠═════╬════════════════╣
║  1  ║ 1              ║
║  2  ║ 1,2            ║
║  6  ║ 1,2,3,6        ║
║  9  ║ 1,3,9          ║
║ 99  ║ 1,3,9,11,33,99 ║
╚═════╩════════════════╝