Hat-Trik,
Примеры (+/-)
// избыточный код
$res = mysql_query('select id_diary from diary_comment where id_diary=1');
while ($row = mysql_fetch_assoc($res)) {
$ids[] = $row['id_diary'];
}
mysql_query('delete from diary_comment where id_diary in(' . join(',', $ids) . ')');
mysql_query('delete from diary where id=1');
// боле менее компактно
mysql_query('delete from diary where id=1');
mysql_query('delete from diary_comment where id_diary not in(select id from diary)');