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The Entity-Relationship Modeling Technique: attributes

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• Attributes (of an entity)
  • Entities have attributes

    Example:

    You (an entity) has a SSN and name        The Social Security Number and Name are examples of attributes

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  • Each attribute can take on values from a certain domain.

  • Example:

    The entity person has the attribute Social Security Number. This attribute can assume the values between 000-00-0000 and 999-99-9999

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• Types of Attributes

  Attributes can come in different types....

  • Simple attributes: the attribute only takes on atomic values.
    Example simple attributes:

    color, age, salary

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  • Compound attributes: the attribute takes on structured values.
    Example compound attributes:

    address (consists of street, city, zip), name (consists of first_name, middle_initial, last_name)

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  • Multivalued attributes: the attribute takes on a set of values.
    Example multivalued attribute:

    earned_degree (can take on {BS, MS} - a set; because a person can earn multiple degrees)

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  • You can also nest any of the above types of attributes.

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• Special kinds of attributes
  • Derived attributes:   values of a derived attribute can be derived (= computed) from other attributes.

    Example:

    age - can be derived from "birth date".

    Note:

    Never store a derived attribute in the database, you introduce redundance....

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  • Key attributes:   a set of (i.e., one or more) attributes for which no two different entities will have the same values for that set of attributes.

    Important fact:

    Values of the key attributes can be used to identify an entity uniquely

    Example:

    Social security number is a key attribute and it is commonly used for identifying humans in the US

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• Special Attribute value: The NULL value
  • There is a special value that an attribute can take on: NULL

  • When an attribute value is NULL, its meaning can be:

    The attribute value is not available, or The attribute value is not applicable

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  • IMPORTANT NOTE: (comparing 2 NULL values)

    When comparing whether two attribute values are equal, and the two attribute values are both equal to NULL, the outcome of the compare is FALSE !!!

    I.e.:

    NULL == NULL ===> result is FALSE

    (because NULL means "unavailable", so we don't know the outcome of the equality comparision, thus resulting is FALSE)

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• NOTE: When is an entity not an entity but an attribute...
  • There is a fine line on whether one would call something an entity or an attribute

    E.g., A tire compnay Good-year will call a tire an entity, because it manufacture tires and will need to maintain a lot of information on tires (e.g., how (components) to make it, where it is made, etc - these information will be needed to compute cost/earning for the company)

    On the other hand, a car manufacturer - like Honda - would most likely look at a tire as an attribute of a car : it does not care how or where the tire is made, all Honda needs to know is the make of the tire

    E.g., THIS Accord car has four Michellin 140 tires

    THAT Accord car has four Good-Year 700 tires

  • A rule of thumb is that if something is important enough for the user, and you need to maintain more than the identifying information of the object, then the object is is an entity in the ER-model....

    Otherwise, the object is modeled as an attribute