I think I can tell you the answer: you have a very slow system test that spins up a whole real database, sets up all the tables and stored procedures, and fires

I think I can tell you the answer: you have a very slow system test that spins up a whole real database, sets up all the tables and stored procedures, and fires a load of requests through the system and inspects the responses and the final state of the system. That works; it's just maybe thousands of times slower than unit tests, and much more work to create and maintain.