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Database Fundamentals. The Data Model: More About Relationships. Microsoft Enterprise Consortium: http://enterprise.waltoncollege.uark.edu Microsoft Faculty Connection/Faculty Resource Center http://www.facultyresourcecenter.com. Things you should know ….

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database fundamentals

Database Fundamentals

The Data Model: More About Relationships

Microsoft Enterprise Consortium: http://enterprise.waltoncollege.uark.edu

Microsoft Faculty Connection/Faculty Resource Center http://www.facultyresourcecenter.com

things you should know
Things you should know …
  • This presentation assumes you are already familiar with these terms:
    • Entity and entity type
    • Entity instance
    • Attribute
      • Identifier
      • Multivalued
    • Cardinality
relationships and cardinality
Relationships and cardinality
  • What do the relationships symbols really mean to us? Here are a few examples of how to read the minimum and maximum symbols.
  • A customer can have a minimum of zero orders.
  • A customer can have a maximum of many orders.
  • An order is for a minimum of one customer.
  • An order is for a maximum of one customer.

CUSTOMER

ORDER

LOCATION

EMPLOYEE

relationships
Relationships
  • Based on maximum cardinality, a relationship falls into one of three types:
    • one-to-many
    • many-to-many
    • one-to-one

one-to-many

many-to-many

CUSTOMER

ORDER

INVENTORY

one-to-one

LOCATION

EMPLOYEE

maximum cardinality
Maximum Cardinality
  • Keep in mind that the 3 relationship types (1-to-1, 1-to-many, and many-to-many) describe the maximum cardinality at both ends of a relationship.
  • It’s incorrect to read the minimum and maximum cardinality as “one-to-many”, for instance.

This is NOT one-to-many

This is many-to-many.

ORDER

INVENTORY

This is NOT one-to-one

This is one-to-many.

CUSTOMER

ORDER

degree of the relationship unary
Degree of the relationship - Unary
  • The degree of the relationship refers to the number of entities in a single relationship.
  • The options are: unary, binary, ternary.
  • A unary relationship means a entity is related to itself. There is only one entity.

Supervises

Any one employee—Casey Wilson, for example—may be supervised by a minimum of zero employees.

Any one employee—Casey Wilson, for example—may be supervised by a maximum of one employee.

Any one employee—Carla Stevens, for example—may supervise a minimum of zero employees.

Any one employee—Carla Stevens, for example—may supervise a maximum of many employees.

EMPLOYEE

Supervised by

degree of the relationship binary
Degree of the relationship - Binary
  • A binary relationship means two entities are directly related. This is by far the most common one you see in a data model.

Any one order—order ID # 4528, for example—may have a minimum of one inventory items in the order.

Any one order—order ID # 4528, for example—may have a maximum of many inventory items in the order.

Any one inventory item—Day-Glow Toothpaste, for example—may be sold in a minimum of zero orders.

Any one inventory item—Day-Glow Toothpaste, for example—may be sold in a maximum of many orders.

sold in

ORDER

INVENTORY

has

degree of the relationship ternary
Degree of the relationship - Ternary
  • A ternary relationship means three entities are directly related. However, this three-way relationship between three entities can be decomposed into binary relationships. Therefore, we won’t spend more time on this term.
we ve learned more about relationships
We’ve learned more about relationships
  • In this presentation we’ve seen that a relationship is one-to-one or one-to-many or many-to-many. This information has important implications for how the database is actually built.
  • We’ve also seen what the “degree of the relationship” means. How many entities are involved in a direct relationship: unary (1 entity), binary (2 entity), or ternary (3 entity)?
resources
Resources
  • Microsoft Enterprise Consortium: http://enterprise.waltoncollege.uark.edu/mec.asp
    • The consortium provides teaching material and large databases donated by some major corporations.
  • Microsoft Faculty Connection—Faculty Resource Center http://www.facultyresourcecenter.com/
    • A wide range of teaching resources are available.