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MBA 664 Database Management. Dave Salisbury [email protected] (email) (web site). Evolution of the E-R Model. Basic E-R Model nearly 25 years old complex data relationships and new database technology have outgrown it in some respects Enhanced E-R Model

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Mba 664 database management l.jpg

MBA 664Database Management

Dave Salisbury

[email protected] (email) (web site)

Evolution of the e r model l.jpg
Evolution of the E-R Model

  • Basic E-R Model

    • nearly 25 years old

    • complex data relationships and new database technology have outgrown it in some respects

  • Enhanced E-R Model

    • a response to the shortcomings of the basic E-R model

    • not universally agreed upon in some respects

    • introduced the supertype/subtype relationship

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Supertype/Subtype Relationships

  • Supertype (example: Employee)

    • a generic entity that has a relationship with one or more subtypes

  • Subtype (example: Manager)

    • a subgrouping of a supertype entity that is meaningful to an organization

    • shares all attributes of its supertype, but also has unique attributes of its own and/or :

    • has relationships with other entities distinct from those of other subtypes

The student example l.jpg
The student example






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Two Rules for When to Use Supertype/Subtypes

Use this type of relationship when either (or both) of the following are present:

  • When there are attributes that apply to some (but not all) of the instances of an entity type

  • When the instances of a subtype participate in a relationship unique to that subtype

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Attribute Inheritance

  • “The property by which subtype entities inherit values of all attributes of the supertype.

  • This important property makes it unnecessary to include supertype attributes redundantly with the subtypes.”

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Supertype/Subtype Example 1

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The process of defining a more general entity type from a set of more specialized entity types

A “bottom-up” approach


The process of defining one or more subtypes of a general entity based on distinguishing attri-butes or relationships

A “top-down” approach

Two Processes to Develop Supertype/Subtypes

Both approaches can be used together

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Addresses the question of whether an instance of a supertype must also be a member of at least one subtype


Addresses the question of whether an instance of a supertype may simultaneously be a member of two (or more) subtypes

Supertype/Subtype Constraints

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Completeness Constraint: Two Possible Rules

  • Total Specialization Rule (Double-Line)

    • Specifies that each entity instance of the supertype must be a member of some subtype in the relationship (Example: all STUDENTS are either UNDERGRADUATE or GRADUATE students)

  • Partial Specialization Rule (Single-line)

    • Specifies that an entity instance of the supertype is allowed to not belong to any subtype (Example: FACULTY and STAFF are not the only possible members of the entity EMPLOYEE)

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Completeness constraint

Total specialization

Partial specialization

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Can an entity instance simultaneously be a member of two or more subtypes?

Disjointed constraint

Disjoint rule

Overlap rule

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Subtype discriminators

Attribute of the supertype whose value determines to which subtype an instance belongs

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Arrangement of super- and subtypes where each subtype has only one supertype.

Supertype/subtype hierarchy

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Another example... only one supertype.