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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 ….

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Database Fundamentals

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  1. 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

  2. Things you should know … • This presentation assumes you are already familiar with these terms: • Entity and entity type • Entity instance • Attribute • Identifier • Multivalued • Cardinality

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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.

  9. 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)?

  10. 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.

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