If I have a relation:
A B C 1 2 3 4 5 6 7 2 8
Where $A\mapsto B$ and $C \mapsto B$. Doesn't that mean that $AC \mapsto B$? How can you even have this relation? What is the primary key?
I'm looking at this slide 10 on this power point: http://www.cs.utah.edu/~lifeifei/cs5530/slides/lecture13.pdf
I would think that if $A$ and $C$ are the same, then $B$ must also be the same. If you can prove this, please show how. The substitute teacher for the day (prof's PhD student) says it can't be done.
EDIT: Wouldn't you just show it by saying:
$AC \mapsto BC$ by augmentation Then, since $C \mapsto B$... you could write $AC \mapsto $B?