Part II: Why did the database administrator go on a diet?
The joke raises the question of why the database decides to go on a diet. The response is humorous, as it mentions having "too many empty calories in the NULL values." Here, an analogy is made between empty calories (calories without nutritional value) in food and NULL values (values without content) in the database.
The underlying idea of the joke is that the database wants to reduce its burden and simplify its structure by getting rid of unnecessary NULL values, much like a person may want to reduce their consumption of empty calories in a diet to become healthier.