$sql = "SELECT col_2 FROM theTable WHERE col_1 = $value_1";
		$sth = $dbh->prepare($sql) or die("Could not prepare!" . $dbh->errstr);
		$sth->execute() or die("Could not execute 1!" . $dbh->errstr);
		while ($data = $sth->fetchrow_array()) {
			push(@col_1, $data);
		}
		$sql = "SELECT col_2 FROM theTable WHERE col_1 = $value_2";
		$sth = $dbh->prepare($sql) or die("Could not prepare!" . $dbh->errstr);
		$sth->execute() or die("Could not execute 2!" . $dbh->errstr);
		while ($data = $sth->fetchrow_array()) {
			push(@col_2, $data);
	}

foreach $e (@col_2) { $union{$e} = 1 }
foreach $e (@col_1) {
    if ( $union{$e} ) { $isect{$e} = 1 }
}
@isect = keys %isect;