Entity relationship models are an important part of designing a database due to the increased complexity of their design. E/R diagrams help the designer conceptualize the design graphically as opposed to trying to visualize and describe it through text only. In 1976, Peter Chen introduced the E/R data model using Chen notation. While he did not invent this concept, his paper helped to standardize and popularize the model.
In an Entity relationship diagram (ERD), graphical representations are used to embody database components. Included in these diagrams are entities, attributes, relationships, and cardinalities. Each entity is described by a set of attributes that identify characteristics about the entity. Relationships between entities describe associations among the data. Relationships can be thought of as verbs, linking two or more nouns. Cardinalities represent the number participating in the relationship. Three types of relationship cardinalities can exist: one-to-many (1:M), many-to-many (M:N), and one-to-one (1:1).
In a Chen notation diagram, entities are represented by rectangles with the entity name written inside. Relationships are represented by a triangle. Two entities can be connected with a line between them and their respective cardinalities written were the line connects each entity. An example of entities and relationships in written form is “one cruise ship carries many passengers.” In this instance, many passengers can only be carried by one cruise ship.
Another type of diagram is known as Crows Foot notation which is derived from Chen notation. It is very similar to Chen notation and the only real differences are in the way that relationships are represented by symbols instead of letter or numbers. Fundamentally however, they are the same.