Logical Data Modeling - Association

About

An association is a relationship between two or more entities. It's represented by a link in the data model.

An association:

• answers to the question why two entity need to know about one another.
• specifies a semantic relationship (ie is … of) that can occur between entities.

Articles Related

Example

• A mathematical function y = f(x) creates an association relationship between the set of x and the set of y

• A book has been written by one or many authors.

Properties

The following relationship properties are visible:

• role
• multiplicity
• direction

And an association may also have a type information added with a symbol at one of its end such as:

• composition is a black diamond
• dependency is a dashed arrow
• aggregation is a hollow diamond (at the aggregate end)

Visualization

Entity Diagram

Example of Entity Diagram

where:

• PublishedBy is an association where
  • 1 is the multiplicity of the publisher end and indicates that a book is published by one publisher
  • * is the multiplicity of the book end and indicates that a publisher publishes many books
• WrittenBy is an association
  • * is the multiplicity of the author end and indicates that a book is written by many Author
  • * is the multiplicity of the book end and indicates that a author writes many books

Class Diagram

Object - Class Diagram (UML)

• Any Association is drawn as a diamond with a solid line to the entity (class).
• The Association’s name can be shown as a name string near the Association symbol
• An Association end is the connection between the line depicting an Association and the icon (often a box) depicting the class.
• The multiplicity attached to a class end denotes the possible cardinalities of the set of entity.

The figure shows:

• a binary association: where the solid triangle indicates the order of reading: Player PlayedInYear Year
• a ternary association: between Team, Year, and Player with id (ends) named team, season, and goalkeeper (goalie) respectively.