I was reading my notes, and I came across this:
Another example would be to find the names of the lecturers teaching all modules. We might reach for the following incorrect expression:
{<N> | ∃M ∀C ∀MN ∀Cr (lecturer(M, N) ∧ module(C, MN, Cr) ∧ teach(M, C))}
Instead, the following is more accurate:
{<N> | ∃M ∀C ∀MN ∀Cr (lecturer(M, N) ∧ (module(C, MN, Cr) → teach(M, C)))}
The first one states that as long as there exists a lecturer who has taught a module that exists in the database, the name will be returned (which is not what we want). Or at least that is how I interpreted this.
Is there any reason why the first expression is incorrect and (more importantly) the second one is correct?
