php pdo如何多层查询（带返回值的查询）？

$row=$pdo->prepare（select * from A） //A表里有多条数据
$row->execute();
while($rs=$row->fetch()){
echo $rs[aid];
foreach ($pdo->query('select * from B where pid=’.$rs['id'].' order by bid asc') as $res) {
echo $res['bid'];
}
}
这是取多个

取单个的
$row=$pdo->prepare（select * from A） //A表里有多条数据
$row->execute();
while($rs=$row->fetch()){
echo $rs[aid];
$re=$pdo->query('select * from B where pid='.$rs['id'].' order by bid asc');
$res=$re->fetch();
echo $res['bid'];
}

用foreach循环