How to do unit testing and how it is different from integration testing in frameworks like Django and fastapi?

Let's say I have a Fastapi application like this (This code is taken from documentations):

app = FastAPI()

@app.get("/foo")
async def read_main():
    return {"msg": "Hello World"}

I believe there are two ways of testing this view. The first one is by the use of a client object. For instance:

client = TestClient(app)

def test_read_main():
    response = client.get("/")
    assert response.status_code == 200
    assert response.json() == {"msg": "Hello World"}

However, I think this is not a unit test. It is more like an integration test. Because we are actually running fastapi codes as well. The second option which in my opinion is closer to unit test definition is to run the function directly:

def test_read_main():
    response = read_main()
    assert response == {"msg": "Hello World"}

Similarly, in Django, we can directly call the view function, or by using a client object. My first question is which one is better?

Let's say we chose one of them and now in order to prevent calling the database I have mocked the database. Now I want to test a view that just checks if something exists in the database. My second question is what is the point of testing such view? Becuase after mocking the database, we know what would happen when calling the database with given arguments.

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