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Relationship Transferability

Objectives

This tutorial covers the following objectives:

  • Describe and give an example of relationship transferability
  • Understand the difference between transferable and nontransferable relationships
  • Illustrate nontransferable relationships on ERDs

Purpose

  • Once a class has been allocated to a teacher, can that class later be transferred to another teacher, possibly in mid-semester?
  • Usually yes, because if not, what would we do if the original teacher becomes sick?
  • Some health clubs allow memberships to be transferred from one person to another, but other health clubs don’t.
  • This business rule is normally determined by what is most efficient and most profitable for the club.

Relationship Review

Relationship Review
  • Let’s review a simple relationship between EMPLOYEE and DEPARTMENT.

Optionality:

  • Must every EMPLOYEE be assigned to a DEPARTMENT?
  • Must every DEPARTMENT be responsible for an employee?

Question: Can you have an EMPLOYEE not assigned to a DEPARTMENT?

ANSWER: NO, so relationship is drawn as mandatory (solid line).

Question: Can you have a DEPARTMENT not responsible for any EMPLOYEEs?

ANSWER: It depends on the business rules of the company in question, but in this case YES “We have some departments that currently have no employees assigned to them”, so relationship is drawn as optional (dashed line).

Cardinality:

  • How many EMPLOYEEs can a DEPARTMENT be responsible for?
  • How many DEPARTMENTs can an EMPLOYEE be assigned to?

ANSWER: Each DEPARTMENT is responsible for one or more employees, so we draw a crows foot at the EMPLOYEE side of the relationship. Each EMPLOYEE can be assigned to only one DEPARTMENT , so we draw a single toe at the DEPARTMENT
side of the relationship

Transferability:

  • Can an EMPLOYEE be transferred from one DEPARTMENT to another DEPARTMENT?

ANSWER: yes, an EMPLOYEE and change from one DEPARTMENT to another. This makes the relationship transferable.

Relationship Transferability

Transferable Relationship

  • Transferable: A STUDENT being allowed to move from one STUDY GROUP to another.
  • There is a relationship between STUDENT and STUDY GROUP that is transferable.
Relationship Transferability

Nontransferable Relationship

  • Nontransferable: A STUDENT can be issued a RECEIPT for paying tuition fees, taking a certification exam, or purchasing items at the bookstore.
  • Once a RECEIPT has been issued, it cannot be transferred to another STUDENT.
Nontransferable Relationship
  • If it was issued in error, it would have to be cancelled, and another RECEIPT would have to be written up.
  • The relationship between STUDENT and RECEIPT is nontransferable.

Nontransferable: If the transferability of a relationship is false, the relationship is said to be nontransferable. Nontransferable relationships are important to note because they usually reflect a business rule and will have implications when the ERD is implemented as a database.

More Nontransferable Relationships

  • Ownership of a POEM belongs with its AUTHOR.
  • Authorship is a relationship that cannot be moved to another person.
Nontransferable Relationships

Terminology

Key terms used in this tutorial included:

  • Nontransferable
  • Transferable

Summary

In this tutorial, you should have learned how to:

  • Describe and give an example of relationship transferability
  • Understand the difference between transferable and nontransferable relationships
  • Illustrate nontransferable relationships on ERDs

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