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Understanding Database Relationships: Fundamentals and Cardinality

Learn the basics of defining relationships in the data model, including cardinality and degrees of relationships. Walk through examples to understand minimum and maximum cardinalities from both entities’ perspectives.

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Understanding Database Relationships: Fundamentals and Cardinality

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  1. Database Fundamentals Defining Relationships in the Data Model 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 that you are familiar with these terms: • Entity, attribute, identifier, multivalued attribute, relationship, cardinality • One-to-many, many-to-many, one-to-one • Degree of the relationship: unary, binary, ternary

  3. Cardinality: Define a relationship • In a relationship between two entities (or an entity related to itself), the minimum and maximum cardinalities are defined by asking two questions from each end of the relationship. • In the example below you must determine the minimum and maximum cardinality from the “customer” side for its relationship to order. • Next, you must determine the minimum and maximum cardinality from the “order” side for its relationship to customer. Q2 Q4 Q3 Q1 CUSTOMER ORDER

  4. Define a relationship: 2 questions • Let’s walk through a detailed example of defining the relationship. • Defining CUSTOMER’s relationship to ORDER. • Q1 Minimum cardinality: For any one customer—Sylvia Sanchez, for example—what is the minimum number of orders allowed? • Q2 Maximum cardinality: For any one customer—Sylvia Sanchez, for example—what is the maximum number of orders allowed? CUSTOMER ORDER

  5. Define a relationship: 2 more questions • Defining ORDER’s relationship to CUSTOMER. • Q3 Minimum cardinality: For any one order—order # 2573, for example—what is the minimum number of customers allowed? • Q4 Maximum cardinality: For any one order—order # 2573, for example—what is the maximum number of customers allowed? CUSTOMER ORDER

  6. Reading the relationship • How would you read the relationship shown below? You can interpret the relationship based on the way you know it was defined. • Any one department, such as marketing, can have a minimum of zero employees. • Any one department, such as marketing, can have a maximum of many employees. • Any one employee, such as Jack Jenkins, can have a minimum of one departments. • Any one employee, such as Jack Jenkins, can have a maximum of one departments. DEPARTMENT EMPLOYEE

  7. What was covered … • This presentation has shown how you define the relationship between two entities. • There are two questions to ask from each side of the relationship—a total of 4 questions.

  8. 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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