Supertypes and Subtypes

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Supertypes and Subtypes

• Material drawn from
• Hoffer, J.A., Prescott, M.B., and McFadden, F.R.,
• Modern Database Management, 6th edition.
• Upper Saddle River, NJ Pearson Education, 2002.

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Supertypes and Subtypes

• One of the major challenges in data modeling is
  to recognize and clearly represent entities that
  are almost the same that is, entity types that
  share common properties but also have one or more
  distinct properties that are of interest to the
  organization.
• Supertype
• Subtype
• ?

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Attributes

• Subtypes inherit all supertype attributes
• Subtypes have attributes that are different from
  other subtypes
• ?

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All employee subtypes will have emp nbr, name,
address, and date-hired
Each employee subtype will also have its own
attributes
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When to use Super/Subtypes

• Attributes apply to some but not all instances of
  an entity
• Instances of a subtype participate in
  relationships unique to that subtype
• Generalization and specialization
• generalization from sub to super
• specialization from super to sub
• ?

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Both outpatients and resident patients are cared
for by a responsible physician
Only resident patients are assigned to a bed
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All these types of vehicles have common attributes
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So we put the shared attributes in a supertype
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Applies only to purchased parts
Only applies to manufactured parts
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Created 2 subtypes
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Constraints

• Completeness
• must every supertype member also be a subtype
  member?
• total or partial specialization
• Disjointedness
• can an instance of a supertype be in more than
  one subtype?
• disjoint or overlap
• subtype discriminator
• Boolean attribute
• ?

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Hierarchies

• Subtypes can be supertypes as well
• a subtype is subordinate to one and only one
  supertype a supertype can have many subtypes
• Attributes are assigned at the highest possible
  level
• Subtypes inherit attributes from all supertypes
  above them in the hierarchy
• ?

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