Missing data, and the null that is data

Reaching past what is there gives no value, and a rule derives no tuple where one of its expressions has none. Reaching a key that is present and null gives the null value, and the row stays:

So the two are distinguishable, which is the point: null is one of the value shapes and an ordinary value, while an absent key has no value at all. Ask for the rows an expression lost with not defined(e).