Entity types are the core modeling constructs in an ER model. They represent real or abstract things or concepts and cannot be detailed by other entity types or relationship types. But, entity types are related to other entity types (via relationship types) and they have attributes which represent their properties (see: eERM attribute allocation diagram).
Entity types are represented by rectangles (see Figure 1).
Figure 1: Examples for entity types
Please note: An entity type can be detailed by only one eERM attribute allocation diagram.
Relationship types
Entity types are usually associated with other entity types. In Chen´s ER model - as well in the ARIS eERM - these associations are represented by an object type of its own: relationship type (see Figure 2). The connection type “defines” is used to assign the participating entity types.
Figure 2: Relationship types in the eERM
Besides the diamond, ARIS provides another relationship type symbol, the so-called “re-interpreted” relationship type. This symbol of is a diamond (-> relationship type) inside a rectangle (-> entity type).
This symbol is used if a relationship type itself participates in another relationship type. In this case, it acts as an entity type (see Figure 3).
Figure 3: Re-interpreted relationship type in the eERM
Like entity types relationship types cannot be detailed by other entity or relationship types, and they also may have attributes.
Please note: A relationship type can be detailed by only one eERM attribute allocation diagram.
Relationship types, however, cannot exist “stand alone”, there must be at least 2 entity types assigned to it.
Relationship types are typically differentiated in 3 basic types:
- 1:n relationship types
- 1:1 relationship types
- n:m relationship types
In most data modeling projects (min, max)-cardinalities are used to describe relationship types more precisely and concisely. Cardinalities are discussed later in a separate article.