Logical Data Modeling

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:

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)

As the term association design a visual representation of an relationship, the term association is also sometimes combined with the relationship properties.

Example:

Directed Aggregation Association can be read a asymmetric (directed) aggregation relationship

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.