- 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.
- 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.
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:
Example of Entity Diagram
- PublishedBy is an association where
- WrittenBy is an association
- 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 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.