RustyNail

 select distinct substring_index(substring_index(a.m, ','
                                             , b.help_topic_id + 1), ',', -1) m,
                         a.f
         from mm a
                  join
              mysql.help_topic b
              on b.help_topic_id < (length(a.m) - length(replace(a.m, ',', '')) + 1)
         where type = 2
         
         // m -> 1,2,3