Order items in MySQL by a fixed list?

You can do that by using either:

ORDER BY FIND_IN_SET(id, '3,11,7,1')

or

ORDER BY FIELD(id, 3, 11, 7, 1)

or

ORDER BY CASE id WHEN 3 THEN 0
                WHEN 11 THEN 1
                 WHEN 7 THEN 2
                 WHEN 1 THEN 3
                        ELSE 4
         END