You can MySQL do the checking for you:

$dbh->do('insert into foo values (x,y,z)');
if ($dbh->err && $dbh->err == 1062) {
   # we've got a duplicate
}
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If you wanted to replace an existing record, look up the syntax for REPLACE where you can do something like:

$dbh->do('replace foo values (x,y,z)');
[download]

Hope this helps...

gav^