trouble with two databases and huge data

[Joseph Puths]

hi all,
I am working on LAMP, I want to compare datas (say serial number) between two databases,
i am taking serial number from database A and store in a single variable called $numbers, whose code is like

$Query1="SELECT serno
FROM batch,company,card
WHERE batch.batch=card.batch
ORDER BY card.serno";
$cntr=1;
$numbers="";
$db->query($Query1);
$numRows1=$db->numRows();
while($reslt = $db->fetchRow())
{
if($cntr!=$numRows1)
{
$numbers=$numbers."".$reslt[0].",";
}
else
{
$numbers=$numbers."".$reslt[0];
}
$cntr++;

}

And i am querying in the second database B as like,
"SELECT ----------,--------,--------
FROM table
WHERE CardSerialNumber IN($numbers) group by BEGIN_DATE ";

The code is working fine for low quantity of data, that means the result set of Ist query is less than 20000.
But whenever the resultset of first query exceeds more than 2 lakh.. the code wont work...
Any solutions....
Thanks in advance..