Re: Please help me with Relational Database design

From: Jan Hidders <>
Date: 5 Mar 2003 23:26:14 +0100
Message-ID: <>

_at_s wrote:
>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 of reflexivity we can always, i.e., no matter what other FDs we already derived, add:

(3) BC -> B

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
Received on Wed Mar 05 2003 - 23:26:14 CET

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