我收到一些错误xpression＃9的SELECT列表不在GROUP BY子句中并且包含非聚合列

Expression #9 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'nilai.b.id_nilai' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

$x['sudah']=$this->db->query("select * from siswa a, nilai b where a.id_siswa=b.id_siswa group by b.id_siswa");