code: def select_join(self, table, field, join_table, join_field):
          
          sql = "select {table}.{field} from {table} left join {join_table} "\
                "on {table}.{field}={join_table}.{join_field} "\
                "where {join_table}.{join_field} is null;".format(table=table,
                                                                  field=field,
                                                                  join_table=join_table,
                                                                  join_field=join_field)
          return super(PGWrapper, self).execute(sql, result=True).results
docs: .. :py:method::
  
          Usage::
  
              >>> select_join('hospital', 'id', 'department', 'hospid')
              select hospital.id from hospital left join department on hospital.id=department.hospid where department.hospid is null;