Присоединяйтесь к JSON Field

  SELECT 
       user.user_id, 
       user.user_name, 
       user_group.user_group_id,
       user_group.group_name
   FROM user
   LEFT JOIN user_group on JSON_CONTAINS(user.user_groups, CAST(user_group.user_group_id as JSON), '$')
PDXfoster