[dillydally]

I didn't read the part about validations. That's silly.

Even if you have to do it in the application layer, why would you need more than one SELECT? Just get the installed apps from the DB in a single query and do a set difference operation to only get the apps not already installed.

But I agree that putting the constraint in the DB layer is the correct solution. With MySQL you can just do INSERT IGNORE to only add apps not yet marked as being installed.