$query1="(select Name from Customers where CustId=?)";
$query2="(select Name from Sales where SalesId=?)";

$sth1=$dbh->prepare($query1);
$sth2=$dbh->prepare($query2);

if ($x>10) {
 $sth1->execute($custid);
}
else {
 $sth2->execute($salesid);
}