SELECT screen_name, substring(member_since,1,12) from users where substring(member_since,1,12) in (SELECT substring(member_since,1,12)
   FROM users
   GROUP BY substring(member_since,1,12)
   HAVING COUNT(*)>50)


return:
Expression #1 of HAVING clause is not in GROUP BY clause and contains nonaggregated column 'Twitter.users.member_since' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by