Generalization and Specialization

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Generalization and Specialization

• Generalization The process of defining a more
  general entity type from a set of more
  specialized entity types.
• Specialization The process of defining one or
  more subtypes of the supertype, forming
  supertype/subtype relationships.

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Specialization Example
Start with 1 entity type
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Specialization Example
Specialize to 1 super and 2 subtypes
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Generalization Example
Start with 3 entity types
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Generalization Example
Generalize to 1 entity type with 2 subtypes
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Supertypes and Subtypes

• Subtype A subgrouping of the entities in an
  entity type which has attributes that are
  distinct from those in other subgroupings.
• Supertype An entity type whose subtypes share
  common attributes. Attributes that are shared by
  all entities (including the identifier) are
  associated with the supertype.

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Example Supertype Subtypes
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Subtype Entities

• Supertype entity
• generic entity thats divided into subtypes
• Subtype entity
• subset of a supertype entity that shares common
  attributes or relationships distinct from other
  subsets
• Exclusive relationship
• subtypes of a supertype are mutually exclusive,
  and each instance of the supertype is categorized
  as exactly one subtype

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Example Exclusive Relationship
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Subtype Entities (contd)

• Exclusive Subtypes - mutually exclusive subtypes,
  i.e. each instance of the supertype is required
  to be a member of exactly one subtype.
• Nonexclusive Subtypes - subtypes may overlap, and
  an instance of the supertype may simultaneously
  belong to more than one subtype

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Subtype Entities (contd)

• Exhaustive - all subtypes are defined for a
  supertype
• Nonexhaustive - some (but not all) of the subtype
  have been defined for a supertype. (indicated by
  a blank rectangle)
• Inheritance the property by which all
  attributes of a supertype become attributes of
  the subtype

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Exclusive, Nonexhaustive Subtypes
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Nonexclusive, Nonexhaustive Subtypes
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Deciding to use a Super/Subtype

• Use when entity instances share some attributes,
  but some instances
• have special attributes, and/or
• are involved in special relationships
• that other instances do not have/are not involved
  in

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Attribute Inheritance

• Subtype entities inherit values of all attributes
  of the supertype.
• An occurrence of a subtype is also an occurrence
  of the supertype.

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Use of Supertype/Subtype

• There are attributes that apply to some (but not
  all) of the instances of an entity type.
• The instances of a subtype participate in a
  relationship unique to that subtype.

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Constraints in Supertype/ Subtype Relationships

• Completeness Constraints
• Whether an instance of a supertype must also be a
  member of at least one subtype.
• Total Specialization Rule Yes
• Partial Specialization Rule No

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Constraints in Supertype/ Subtype Relationships

• Disjointness Constraints Whether an instance of
  a supertype may simultaneously be a member of two
  (or more) subtypes.
• Disjoint Rule No
• Overlap Rule Yes

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Total Specialization Disjoint Example
Supertype can only be 1 subtype
Supertype MUST be at least 1 subtype
Special relationship
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Total Specialization Overlap Example
Supertype can be more than 1 subtype
Special relationship
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Partial Specialization Example
Supertype does NOT have to be a subtype
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Domain

• set of all data types and ranges of values that
  attributes may assume
• specifies the characteristics of attributes data
  type, length, format, range, allowable values,
  meaning, uniqueness, and null support.

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Advantages in using Domains

• Domains verify that the values for an attribute
  are valid
• Domains ensure that various data manipulation
  operations are logical.
• Domains help conserve effort in describing
  attribute characteristics.

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UML

• UML Unified Modeling Language
• UML has many components to graphically model
  different aspects of an entire software system
• UML Class Diagrams correspond to E-R Diagram, but
  several differences.

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Summary of UML Class Diagram Notation
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UML Class Diagrams (Contd.)

• Entity sets are shown as boxes, and attributes
  are shown within the box, rather than as
  separate ellipses in E-R diagrams.
• Binary relationship sets are represented in UML
  by just drawing a line connecting the entity
  sets. The relationship set name is written
  adjacent to the line.
• The role played by an entity set in a
  relationship set may also be specified by writing
  the role name on the line, adjacent to the entity
  set.

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UML Class Diagrams (Contd.)

• The relationship set name may alternatively be
  written in a box, along with attributes of the
  relationship set, and the box is connected, using
  a dotted line, to the line depicting the
  relationship set.
• Non-binary relationships drawn using diamonds,
  just as in ER diagrams

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UML Class Diagram Notation (Cont.)
overlapping
disjoint
Note reversal of position in cardinality
constraint depiction Generalization can use
merged or separate arrows independent of
disjoint/overlapping
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UML Class Diagrams (Contd.)

• Cardinality constraints are specified in the form
  l..h, where l denotes the minimum and h the
  maximum number of relationships an entity can
  participate in.
• Beware the positioning of the constraints is
  exactly the reverse of the positioning of
  constraints in E-R diagrams.

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UML Class Diagrams (Contd.)

• The constraint 0.. on the E2 side and 0..1 on
  the E1 side means that each E2 entity can
  participate in at most one relationship, whereas
  each E1 entity can participate in many
  relationships in other words, the relationship
  is many to one from E2 to E1.
• Single values, such as 1 or may be written on
  edges The single value 1 on an edge is treated
  as equivalent to 1..1, while is equivalent to
  0...

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SUMMARY

• This course described the entity- relationship
  (E-R) model and the use of this tool within the
  context of logical data modeling. The model has
  been extended to include constructs such as
  subtypes and supertypes. However, at the present
  time there is no standard notation for E-R
  modeling.
• The basic constructs of entity-relationship
  modeling are entity types, relationships, and
  associated attributes.