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General doubt dbms
Shivani gaikawad
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Databases
Sep 14, 2018
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An instance of a relational schema R(A,B,C,D) has distinct values for attribute A then A can be
1) candidate key
2)non key
3)super key
4)none
databases
candidate-key
Shivani gaikawad
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Sep 14, 2018
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Shivani gaikawad
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Any attribute that can be used to uniquely identify a row can be a super key and a minimal super key is called as candidate key.
Since the key here has only one attribute, it is a candidate key.
goxul
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Sep 14, 2018
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Shaik Masthan
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Sep 14, 2018
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i too notice hat, But keys and Functional Dependencies can't be said to Particular instances.
By knowing Keys and Functional Dependencies, we have to take instances.
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manisha11
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Sep 14, 2018
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for a given instance, it should be a foreign key.
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Prince Sindhiya
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Sep 15, 2018
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@shaikh sir you are right
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Here ,we may say that A->BCD ,FD holds for this instance
And $A^+={A,B,C,D}$
But FD s are defined on the schema itself not the instance, so based on the state of the instance
we cannot say what holds for schema (there can be a many instances for R).
Prince Sindhiya
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Sep 15, 2018
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