Re: Please help me with Relational Database design
Date: 5 Mar 2003 23:26:14 +0100
>Well, I hoped that the Greek letters worked, but it didn't. Please fill in
>for the questiomarks:
>A -> B |- AC -> B
Basically Armstrong's rules tell you how you can derive other functional dependencies (FDs) from the FDs that you already know that hold. So what you can do is start with a certain small set and then use these rules again and again to add all the other FDs that then also hold. In this case you have to start with A -> B and check if at one point in time AC -> B will also be added.
So in this case we start with:
(1) A -> B
Because this FD holds we can use the augmentation rule and add:
(2) AC -> BC
Because (2) and (3) are in the set that we already derived they can be "connected" with the transtivity rule and we can now also add:
(4) AC -> B
So you see that starting with only A -> B we can derive that AC -> B.
- Jan Hidders