MySQL一行变多行，多行变一行

select a.classid, substring_index(substring_index(a.classname, ',', b.help_topic_id + 1), ',', -1) as spitId
from classroom a
       join mysql.help_topic b on b.help_topic_id < (length(a.classname) - length(replace(a.classname, ',', '')) + 1)
where a.classid = 5;

select classname ,group_concat(classid Separator ',') as score from classroom group by classname;