Query to prevent booking overlap - oracle

Question

I'm doing an app in Apex Oracle and trying to find a query that could prevent people from booking a room already booked. I managed to find a query that can prevent picking a date that starts or ends in between the booking time but I can't find how to prevent overlaping. By that I mean if someone books a conference room feb 2nd to feb 5th, someone can book the same room from feb 1st to feb 7th. That is what I'm trying to prevent. Thanks for the help!
Here's my first query
SELECT RES_ID_LOC FROM WER_RES
WHERE (CAST(RES_DATE_ARRIVE AS DATE) < CAST(TRY_RESERVE_START_DATE AS DATE) OR CAST(RES_DATE_DEPART AS DATE)
CAST(TRY_RESERVE_START_DATE AS DATE))
AND (CAST(RES_DATE_ARRIVE AS DATE) < CAST(TRY_RESERVE_END_DATE AS DATE) OR CAST(RES_DATE_DEPART AS DATE) > CAST(TRY_RESERVE_END_DATE AS DATE))

Answer 1

The main issue you'll have here is concurrency, namely (in chronological order)
User 1
runs overlap check query, see Room 5 is free, and inserts a row to book it
User 2
runs overlap check query, see Room 5 is free, and inserts a row to book it
User 1
commits
User 2
commits
and voila! You have a data corruption, even though the code all ran as you expected.
To avoid this, you'll need some way to lock a resource that multiple might want to book. Thus lets say you have a ROOMS table (list of available rooms) and a BOOKINGS table which is a child of ROOM.
Then your logic will need be something like:
select from ROOM where ROOM_NO = :selected_room for update;
This gives someone exclusive access to the room to check for bookings.
Now you can run your overlap check on that room against the BOOKINGS table. If that passes, then you insert your booking and commit the change to release the lock on the ROOMS row.
As an aside, take care with simply casting strings to dates, because you're at the whim of the format mask of the item matching that default of the database. Better to explicitly use a known format mask and TO_DATE