Evolution of the ER Model

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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

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Supertype/Subtype Notation
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The student example
STUDENT
GRAD STUDENT
UNDERGRAD STUDENT
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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
• 1. When there are attributes that apply to some
  (but not all) of the instances of an entity
  type
• 2. 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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Supertype/subtype relationships in a hospital
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Two Processes to Develop Supertype/Subtypes

• Generalization
• The process of defining a more general entity
  type from a set of more specialized entity types
• A bottom-up approach

• Specialization
• The process of defining one or more subtypes of a
  general entity based on distinguishing
  attri-butes or relationships
• A top-down approach

Both approaches can be used together
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Supertype/Subtype Constraints

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

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

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Completeness Constraint Two Possible Rules

• Total Specialization Rule (Double-line notation)
• 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
  notation)
• 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

• Can an entity instance be a member of the
  supertype but no subtype?

• Total specialization
• partial specialization

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Disjointed constraint

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

• 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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What does that look like?
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Yet another example.
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Supertype/subtype hierarchy

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

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Another example...
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Lets practice...

• A bank has three types of accounts checking,
  savings, and loan. The attributes for each
  account are as follows
• CHECKING Account_No, Date_Opened, Balance,
  Service_Charge
• SAVINGS Account_No, Date_Opened, Balance,
  Interest_Rate
• LOAN Account_No, Date_Opened, Balance,
  Interest_Rate, Payment
• Assume that each bank account must be a member of
  at least one of these subtypes.Develop an EER
  model segment to represent this situation.