Superkey, candidate key & primary key

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独厮守ぢ 2021-01-30 03:35

Can any kind soul clarify my doubts with a simple example below and identify the superkey, candidate key and primary key?

I know there are a lot of posts and websites ou

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  • 2021-01-30 03:49

    A superkey is any set of attributes for which the values are guaranteed to be unique for all possible sets of tuples in a table at all times.

    A candidate key is a "minimal" superkey - meaning the smallest subset of superkey attributes which are unique. Removing any attribute from a candidate key would therefore make it non-unique.

    A primary key is just a candidate key. There is no difference between a primary key and any other candidate key.

    It's not really useful to make assumptions about keys based only on a list of attribute names. You need to know what dependencies are supposed to hold between the attributes. Having said that, my guess is that you are right - StudentNumber is likely a candidate key in your example.

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  • 2021-01-30 03:51

    Stretching Cambium's answer, if the PhoneNumber is also unique along with StudentNumber then candidate keys would be:- {StudentNumber},{PhoneNumber}.
    Here we can't assume {StudentNumber,PhoneNumber} as a single candidate key because if we omit one attribute say StudentNumber we still get a unique attribute{PhoneNumber} thus, violating the definition of candidate key.

    Primary key: Choose one candidate key out of all candidate keys. There are 2 candidate keys so we can choose {StudentNumber} as primary key.
    Alternate keys: leftover candidate keys, after choosing primary key from candidate keys, are alternate keys i.e. {PhoneNumber}.

    compound key: a compound key is a key that consists of two or more attributes that uniquely identify an entity occurrence. A simple key is one that has only one attribute. Compound keys may be composed of other unique simple keys and non-key attributes, but may not include another compound key.

    composite key: A composite key contains at least one compound key and one more attribute. Composite keys may also include simple keys and non-key attributes.

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  • 2021-01-30 03:57

    Since you don't want textbook definitions, loosely speaking, a super key is a set of columns that uniquely defines a row.

    This set can have one or more elements, and there can be more than one super key for a table. You usually do this through functional dependencies.

    In your example, I'm assuming:

    StudentNumber    unique
    FamilyName     not unique
    Degree     not unique
    Major      not unique
    Grade      not unique
    PhoneNumber    not unique
    

    In this case, a superkey is any combination that contains the student number.

    So the following are superkeys

    StudentNumber
    StudentNumber, FamilyName
    StudentNumber, FamilyName, Degree
    StudentNumber, FamilyName, Degree, Major
    StudentNumber, FamilyName, Degree, Major, Grade
    StudentNumber, FamilyName, Degree, Major, Grade, PhoneNumber
    StudentNumber, Degree
    StudentNumber, Degree, Major
    StudentNumber, Degree, Major, Grade
    StudentNumber, Degree, Major, Grade, PhoneNumber
    StudentNumber, Major
    StudentNumber, Major, Grade
    StudentNumber, Major, Grade, PhoneNumber
    StudentNumber, Grade
    StudentNumber, Grade, PhoneNumber
    StudentNumber, PhoneNumber
    

    Now assume, if PhoneNumber is unique (who shares phones these days), then the following are also superkeys (in addition to what I've listed above).

    PhoneNumber
    PhoneNumber, Grade, 
    PhoneNumber, Major, Grade
    PhoneNumber, Degree, Major, Grade
    PhoneNumber, FamilyName, Degree, Major, Grade
    PhoneNumber, Major
    PhoneNumber, Degree, Major
    PhoneNumber, FamilyName, Degree, Major
    PhoneNumber, StudentNumber, FamilyName, Degree, Major
    PhoneNumber, Degree
    PhoneNumber, FamilyName, Degree
    PhoneNumber, StudentNumber, FamilyName, Degree
    PhoneNumber, FamilyName
    PhoneNumber, StudentNumber, FamilyName
    

    A candidate key is simply the "shortest" superkey. Going back to the 1st list of superkeys (i.e. phone number isn't unique), the shortest superkey is StudentNumber.

    The primary key is usually just the candidate key.

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