``` sql 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 ```