Not certain about the OP's reasoning, but doing aggregate functions on the database server is often more performant than doing the same in your application. This is, in part, because of the smaller result set sizes.

Also, it's useful having logic baked into the database rather than your application in the case that you need a different application to connect to your database. It's a problem, for example, if you're maintaining your foreign key relationships through ActiveRecords's relations and ignoring it on the server. It makes it much easier for junk data to get in the database.