MySQL - Should I create a different database for users My question isn't really MySQL related, but that's the database I am using. My project was small so I put my user data and my business stuff on the same database. However, it's grown so much since and I've been thinking of separating the user part into an other database. It was nice to have them on the same database since I could do JOIN operations whenever I need to query things. I don't think I can do that on different databases. Are there any other downsides with 2 databases for the same web app? Would connection between them be an issue if they don't live in the same data center, for example.
If the data is related, it should be in the same database unless there is a very compelling reason not to.  By separating the data you have:

Made it impossible to maintain referential integrity
Increased the number of cross-database calls
Increased the amount of maintenance
Increased the likelihood you will build another database for analysis/backend reporting with complex reconciliation rules

If you are looking to improve fault tolerance or increase performance, there are generally better ways to manage that.