select c.*,u.user_name as host_name ,uc.user_name as create_name,(select group_concat(case when real_name is null then user_name else real_name end) from user where INSTR(c.invite_user,concat(',',id,','))>0

order by find_in_set(id,c.invite_user)) as invite

from communicate c left join user u on c.host=u.id left join user uc on c.create_user =uc.id

invite_user字段格式为：,1,5,2,3,

group_concat():逗号分隔，将一列数据

find_in_set():按照指定的ID排序