Is there any tutorial on how to integrate a database for the game? For example, I have a database for the monster data, item data, player data, etc. I want to access it in a room when a player matches and use it for the loop.
Integrating Database (sovled)
I would recommend:
https://github.com/mikro-orm/mikro-orm
Or:
https://github.com/Vincit/objection.js
But none of these are Colyseus related, these will help you to access the data.
In terms of the integration you will have to do your own implementation, no matter what you choose.