原生sql语句执行

public function Text()
    {
        $nation = D("Nation");
        $sqla = "select * from nation";
        $qa = $nation->query($sqla);//查询是query()方法
        $sqlb = "insert into nation values('n015','维族')";
        $qb = $nation->execute($sqlb);//除了查询以外，都是execute()方法

    }