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Super key, key, primary key, candidate key and foreign key...

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• The various "Key" concepts in Relational Database
  • You already know what "key" attributes are from the ER-model: click here

    The key concept in the relational model is defined in an "unusually" difficult manner (well, in a Mathematical manner)...

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  • Super key:

    Superkey = a set of attributes in a relation R such that no 2 different tuples will have the same values for that set of attributes

    Mathematically speaking:

    A set of attribute SK is a superkey if and only if: ∀   t1, t2   in   R:     t1[SK] ≠ t2[SK]

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  • Key:

    Key: is a minimal set of attributes in a relation R such that no 2 tuples will have the same values (for that set of attributes)

    I.e., a key is a minimal superkey.

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  • Candidate key:

    Candidate key = any key

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  • Primary key:

    Primary key = the key that is chosen to be used to identify tuples in a relation.

    Once you have chosen the primary key for some relation, you must use that primary key throughout the database (or else, you database career life will be very miserable)

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  • Foreign key:

    Foreign key = a set of attributes inside some relation R1 that is the primary key of another relation R2

    Example:

    ID in GradeReport is a foreign key:

    The attribute ID in the relation GradeReport (= R1) is the primary key in relation Student (R2)

    CID in GradeReport is also a foreign key:

    The attribute CID in the relation GradeReport (= R1) is the primary key in relation Course (R2)