2

I got relation:

R = { (ABCDEF) , (A B F -> C, B C -> F, F -> A, F -> B, D E -> E, E -> D)}

keys of this relation are:

{BCE}, {EF}

But I don't get it... why? Why not only {EF}.. it got less attributes than {BCE}..

Another example, relation:

R2 = {(ABCDEF), (A -> B C , B -> D , E -> F)}

and the key is:

{AE}

why only one? Why not:

{AE}, {ABE}

ABE also determines all other attributes, why is it not a key?

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    In the 2nd question, ABCDEF also determines all attributes. Why not that too? – ypercubeᵀᴹ Jan 19 '14 at 17:57
  • Yea, I know it determines all atributes. And I ask myself the same question. Why not? Thats what i am asking. Maybe its the dumbest question I have ever asked.. but there it is. – Ariel Grabijas Jan 19 '14 at 18:04
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    Revisit your notes about the definition of a key. See also the Wikipedia page on Superkey where what you call a key (if I understand you correctly) is referred to as a "candidate key" or "minimal superkey". The key word (sorry for the pun) is minimal. – ypercubeᵀᴹ Jan 19 '14 at 18:15
  • I understand why EF is a key (first example) - its obvious. There are no keys among 1-attribute expressions, and EF is a key among 2-attribute expressions. But why BCE is also a key? How can it be minimal if it has 3 attributes, and EF got 2... 2 < 3. Therefore BCE is not minimal. – Ariel Grabijas Jan 19 '14 at 18:39
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    Minimal does not mean smallest in size (number of attributes). But smallest when compared as sets. Means it has no subset which is also a superkey. No subset of EF and no subset of BCE is a superkey, so these two are the minimal superkeys. – ypercubeᵀᴹ Jan 19 '14 at 19:08
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Any relation has multiple keys present in it. The primary key is the minimal key amongst these various candidate keys which we can choose from. Candidate keys are a minimal cover of attributes which determines a tuple. We can have many many candidate keys.

The reason A,B,C,D,E,F,G is a key is since it is not a minimal cover of attributes that determine the entire tuple. In the second example, B is not part of the minimal cover since removing it from the candidate key still makes the remaining attributes a candidate key!

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    Your first sentence does not sound right - how about relations with only one attribute? – dezso Oct 7 '16 at 21:23
  • And theory does not distinguish between primary key and alternate (candidate) keys. This (that we distinguish between the 1 PRIMARY KEY and multiple UNIQUE ones), making the primary somehow more important, is just an implementation choice (made by SQL). – ypercubeᵀᴹ Oct 7 '16 at 23:14

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