Ok, thank you all very much for your quick (!) answers. Really helped me a lot!

Regards,

Fabian

That's a good idea, haven't thought of that.

From your replies I understand it isn't a good idea to create seperate database, so I'll drop that thought.

The only problem I see...

That's because of security issues. With this application users can create and send invoices to their customers. Because all of their sales/accounts/products/... data is stored in that database, we choose...