Entity, Attribute, and Relationship

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Entity, Attribute, and Relationship

• DATA ENTITY TYPE a noun, i.e. roles, events,
  locations, people, tangible things about which we
  wish to maintain data
• ATTRIBUTE noun, data elements that describe
  characteristics of the entity type
• RELATIONSHIP verb phrase, an association between
  entities of interest to the organization
• Labels of the dataflows in DFD's correspond to
  entities and attributes in the ERD.

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ER Modeling Challenges

• Identifying entities and attributes in a syetem
• Entity Student
• Attributes of the Entity
• Student Name
• Social Security No. (Key)
• Major
• Mailbox Number
• Permanent Address
• Street
• City
• State
• Zip
• Credit Hours Earned

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Which of the following would be potential
entities in a system forstudent registration?

• Student
• Registrar
• Course number
• Room
• Class
• Course
• Pre-requisites
• Name
• Junior
• Sophomore
• Jane Stevens
• MIS 321 01
• Instructor
• Password/pin
• Bill

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Potential entities in a system for student
registration . . .

• Entity Types student, course, and depending on
  scope bill, instructor
• Attributes of Entity course number, room, class
  standing, pre-requisites, name, password/pin,
  and depending on scope (instructor)
• Possible Attribute Values junior, sophomore
• Instances Jane Stevens, MIS 321 01

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Types of attributes

• Composite attributes
• Attributes with subattributes
• Address street address, city, state, zip.
• Single valued
• Attribute with only one value
• Age of a person, sudent id
• Multivalued
• Attribute with more than one value
• Tel-phone numbers
• Stored and derived attribute
• Strongly related
• One is derived from other (stored attribute).
• Age of a person can be derived from the attribute
  birth date.
• Null value
• Attribute with no value
• Apartment number in a streetaddress can have the
  value null for an
• address which is not in an appartment

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Criteria for Selecting Primary Key

• The value will not change over the life of
  each instance of the entity
• For each instance of the entity, you are
  guaranteed to have a value for the attribute
• Avoid intelligent keys -- a key whose
  structure indicates classification or location --
  these often change
• Single attribute keys are easier to work
  with than large multi-attribute composite keys
• Create a separate surrogate key

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What would be a good primary key choice for the
following entity?

• ENTITY Course
• Course Number
• Quarter Offered
• Instructor
• Course Description
• Time Offered
• Day Offered

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ID number

• ENTITY VIDEO
• Movie Title , Length , Copy Number ,
  Awards , Lead Actors/Actresses ,
  Availability (logical)
• Movie Title?
• Movie Title and Copy No.?
• How to distinguish between a remake and
  original with the same title?
• May need to create an attribute that is
  unique -- video id number

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Weak Entity Types

• Existence depends on the relation with another
  entity
• No key attribute
• Identified by own attributes in combination with
  the attributes of the entity on which it
• depends on.
• A weak entity cannot exist without the relation
  it has with the entity it is related.

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Symbols in an ERD

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