Database Management

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Database Management Fall 2003 Ternary
relationships and subtyping
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Degree of relationship unary vs. binary vs.
ternary
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Car Rental Agency Example
CUSTOMER
custid
custname
custaddress
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Car Rental Agency Example
Problem where does car rental information belong?
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A ternary relationship
(here, an associative entity implements a
ternary relationship)
(Watson, Chapter 7)
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A non-ternary alternative
LEASE
AUTOMOBILE
AGENT
autoid (FK)
agentid
autoid
custid (FK)
. . .
leasedate (FK)
. . .
agentid
. . .
(this allows for the recording of a lease without
an agent more realistic)
Customer
custid
. . .
(Watson, Chapter 7)
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Data modeling

• A technique for modeling data
• A graphical representation of a database
• The goal is to identify the facts to be stored in
  the database
• Data modeling is a partnership between the client
  and analyst

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The building blocks

• Entity
• Attribute
• Relationship
• Identifier

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Entity types

• Independent
• Dependent
• Associative
• Aggregate
• Subordinate

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Independent

• Often a starting point
• Prominent in the client's mind
• Often related to other independent entities
• Can exist on their own

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Dependent

• Relies on another entity for its existence and
  identification
• Can become independent if given an arbitrary
  identifier

REGION
CITY
regname
cityname
regname
regpop
regarea
citypop
cityarea


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Associative

• A by-product of an mm relationship
• Typically between independent entities
• Can store current or historical data
• Can become independent if given an arbitrary
  identifier

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Aggregate

• Created from several different entities that have
  a common prefix or suffix
• Commonly used with addresses or names

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Subordinate

• An entity with data that can vary among instances

• Map with one table for each entity
• For each of the subtype entities the primary key
  is that of the supertype entity
• You must also make this column a foreign key so
  that a subtype cannot be inserted without the
  presence of the matching supertype

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Generalization, specialization, or subtyping
Watson shows
But well use
(Watson, Chapter 7)