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Databases

Posted by aroshinemunasinghe
aroshinemunasinghe
5,649 Points

Functional Dependency - help

I know there are no course in database called functional dependency which is SAD. So I wonder if you know about functional dependency? please look at this link: https://i.stack.imgur.com/3QD7I.png

and I have got answer:

costumer -> city

costumer -> population

price -> product

price -> costumer

price -> city

price -> population

city -> population

population -> city

but actually the answer is:

(product, costumer) -> price

costumer -> city

city -> population

costumer -> population

:sad: what did I do wrong?!

1 Answer

Kevin D
Kevin D
8,646 Points
Kevin D
Kevin D
8,646 Points

This is a really late answer...but can you also provide an updated image link?

I don't know if this will help you or not, but what I understand from functional dependencies is that the element on the left side of the arrow (leftSideElement -> rightSideElement) is only allowed to be mapped to one unique element on the right side in order to evaluate to true. If that element maps to two different elements, then it evaluates to false. However, two different elements are also allowed to be mapped to the same element (evaluating to true also).

Confused?

I'll try to give an example:

Imagine there are 4 tables called A, B, C and D that list their elements in a single column (Img of what I'm talking about: http://imgur.com/a/GJJSW). A contains the elements a1, a1, a2, a2 B contains the elements b2, b2, b3, b4 C contains the elements c1, c1, c2, c3 D contains the elements d1, d1, d1, d4

If we say A->B, the answer will be false because: a1 -> b2 (a1 works because A1 maps to a unique element) a1 -> b2 a2 -> b3 (a2 doesn't, because it's mapping to 2 different elements) a2 -> b4

If we say C->D, the answer would be true because: c1 -> d1 (c1 works because it is mapping to a unique element) c1 -> d1 c2 -> d1 (c2 also works even though c1 maps to d1 too!) c3 ->d4 (c3 is mapping to d4 and nothing else)

I hope that helps!