Interesting little tip that I must admit never knew was possible (although admitadly, probably isn't needed to be used *that* often).

Suppose you have a shared database of users, and perhaps a small sub-system that has it's own self-contained database, but needs to run queries that include the users in the shared database. Provided the mySQL user has permissions to access both databases, you can simple run a query like:

SELECT database1.sharedUsers.*, database2.userTable.*
FROM database1.sharedUsers
LEFT JOIN database2.userTable.userID ON database1.sharedUsers.userID