SELECT COUNT(`id) as countGuests FROM users_online WHERE id_user IS` NULL; union all SELECT COUNT(`id) as countUsers FROM users_online WHERE id_user IS` NOT NULL;
SELECT COUNT(`id) as countGuests FROM users_online WHERE id_user IS` NULL; union all SELECT COUNT(`id) as countUsers FROM users_online WHERE id_user IS` NOT NULL;
Разве except не производительнее? По сути всё равно 2 запроса, просто у тебя они сливаются, а так 1 будет