This is my query for available rooms in choosen period:

SELECT rooms.room_id 
FROM rooms WHERE rooms.room_id NOT IN 
( 
SELECT reservations.room_id FROM reservations WHERE 
( reservations.arrivaldate >=  $arrival_datetime AND reservations.departuredate <=  $departure_datetime) OR 
( reservations.arrivaldate <=  $arrival_datetime AND reservations.departuredate >=  $arrival_datetime ) OR
( reservations.arrivaldate <=  $departure_datetime AND reservations.departuredate >=  $departure_datetime )
);

How to add average room price column for selected period(from $arrival_datetime to $departure_datetime) from another table (room_prices_table), for every room_id returned from above query. So I need to look in columns whos name is same as room_id...

room_prices_table:
date
room0001
room0002
room0003
...

Something like

SELECT AVG(room0003) FROM room_prices_table WHERE datum IS BETWEEN $arrival_datetime AND $departure_datetime 

??