Complex Query - Optimization

[Robert Hawke]

Hi! Here is a brief description of my problem:

I have a table which displays the occupancy of office rooms.

It has the following layout:
name | room | day | hour

Here is a data example:
Robert | A5 | 1 | 8
Dan | A5 | 1 | 8
Robert | A6 | 1 | 9
Dan | A6 | 1 | 9

The first record means that Robert uses this room every 1st Day of the Week (Monday) from 8 to 9 o clock

Now I want to see which rooms are available (not used) at a certain day at a certain hour.

I already have a working solution, but it is deadly slow (takes about 10 seconds to evaluate), since the table contains about 2000 records.
Has anybody got an idea how to solve this more efficently?

SELECT DISTINCT room from room_schedule WHERE room not in (SELECT DISTINCT room FROM room_schedule WHERE day = 1 and hour = 8) ORDER BY room

Note: This query only works with MySQL 4.1 since older versions do not support Sub-Queries.

Thanx for some ideas!

Robert