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What is the diffrence between Key and Superkey how can we understand ? help me please
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Re: What is the diffrence between Key and Superkey how can we understand ? help me
#2
May 19th, 2008
hi mertucci,
are in doing data modelling?
Superkey is any subset of attributes that uniquely identifies the tuples of a relation. This subset need not be minimal, that is, one can remove some attributes from it and it is still uniquely identifying. If all redundant attributes are removed you get a subset what is called primary key. A key (better: primary key) is the minimal subset of attributes that uniquely identifies the tuples of a relation, that is, you can't remove further attributes from this subset without losing the property of unique Identification. Therefore, the super superkey (like superman) is always the set of all attributes of a relation.
krs,
tesu
are in doing data modelling?
Superkey is any subset of attributes that uniquely identifies the tuples of a relation. This subset need not be minimal, that is, one can remove some attributes from it and it is still uniquely identifying. If all redundant attributes are removed you get a subset what is called primary key. A key (better: primary key) is the minimal subset of attributes that uniquely identifies the tuples of a relation, that is, you can't remove further attributes from this subset without losing the property of unique Identification. Therefore, the super superkey (like superman) is always the set of all attributes of a relation.
krs,
tesu
Last edited by tesuji : May 19th, 2008 at 3:08 am.
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Re: What is the diffrence between Key and Superkey how can we understand ? help me
#3
May 19th, 2008
thanks bro
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Re: What is the diffrence between Key and Superkey how can we understand ? help me
#4
May 21st, 2008
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Originally Posted by tesuji 
...If all redundant attributes are removed you get a subset what is called primary key...
I have to make an extension:
Sometimes there are more than one superkey of a relation. In such case also all redundant attributes have to be removed and the remaining subsets of attributes are then called CANDIDATE KEYS. Now one can freely select ONE of these candidate keys to identify the relation's tuples. In such case, this selected subset is called PRIMARY KEY.
krs,
tesu
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