In his most recent post Michelangelo van Dam talks about unit testing and databases and how, to effectively test what should be tested (the code, not "the ability to fetch data") you need to correctly mock your database objects.

When I state "as is", I truly mean the way it's being used in production. So the database call collects real data on which business logic is applied. You can see this is not a healthy situation, especially when you also have services that apply business logic on data and store it back into the database. In "Chris Hartjes wrote this one sentence that says it all: "Unit test suites are meant to be testing code, not the ability of a database server to return results". And he's right, you shouldn't use database connections when your testing business rules and functional logic.

He goes on to show a few code examples that show a pre-mocked state of testing where the Product information is pulled directly from a PDO connection. The more correct version mocks out this object, though, and overrides the "execute" and "fetchAll" methods to return mocked results.