Entity-relationship Diagram

There are a number of conventions for entity-relationship diagrams (ERDs). The classical notation is described in the remainder of this article, and mainly relates to conceptual modelling. There are a range of notations more typically employed in logical and physical database design, including '' Information Engineering '', '' IDEF1x '' (ICAM DEFinition Language) and '' Dimensional Modelling ''.

COMMON SYMBOLS

An captures how two or more entities are related to one another. Relationships can be thought of (again, roughly) as Verb s. Examples: an ''owns'' relation between a company and a computer, a ''supervises'' relation between an employee and a department, a ''performs'' relation between an artist and a song, a ''proved'' relation between a mathematician and a theorem. Entities are drawn as rectangles, relationships as diamonds.

Entities and relationships can both have attributes. Examples: an employee entity might have a Social Security Number attribute (in the US); the ''proved'' relation may have a ''date'' attribute. Attributes are drawn as ovals connected to their owning entity sets by a line.

Every entity (unless it is a Weak Entity ) must have a minimal set of uniquely identifying attributes. This set is called the entity's Primary Key .

Entity-relationship diagrams don't show single entities or single instances of relations. Rather, they show entity sets and relationship sets (displayed as rectangles and diamonds respectively). Example: a particular song is an entity. The collection of all songs in a database is an entity set. The ''proved'' relationship between Andrew Wiles and Fermat's Last Theorem is a single relationship. The set of all such mathematician-theorem relationships in a database is a relationship set.

Lines are drawn between entity sets and the relationship sets they are involved in. If all entities in an entity set must participate in a relation in the relationship set, a thick line is drawn. This is called a Participation Constraint . If each entity of the entity set can participate in at most one relationship in the relationship set, an arrow is drawn from the entity set to the relationship set. This is called a Key Constraint . To indicate that each entity in the entity set is involved in exactly one relationship, a thick arrow is drawn.

Associative Entity is used to solve the problem of two entities with a many-to-many relationship {Link without Title} .

Unary Relationships - a unary relationship is a relationship between the rows of a single table.

LESS COMMON SYMBOLS

A weak entity is an entity that can't be uniquely identified by its own attributes alone, and therefore must use as its primary key both its own attributes and the primary key of an entity it is related to. A weak entity set is indicated by a bold rectangle (the entity) connected by a bold arrow to a bold diamond (the relationship). Double lines can be used instead of bold ones.

Attributes in an ER model may be indicated as multi-valued or composed. A multi-valued attribute may contain two or more values, and is indicated by a dashed ellipse. A composite attribute may itself contain two or more attributes and is indicated by a double-lined ellipse.

Sometimes two entities are more specific subtypes of a more general type of entity. For example, programmers and marketers might both be types of employees at a software company. To indicate this, a triangle with "ISA" on the inside is drawn. The Superclass is connected to the point on top and the two (or more) Subclass es are connected to the base.

Finally, through Aggregation a relation and all its participating entity sets can be treated as a single entity set for the purpose of taking part in another relation. This is indicated by drawing a dotted rectangle around all aggregated entities and relationships.

CLASSIFICATION

Entity relationship models can be classified in BERMs (Binary Entity Relation Model) and GERMs (General Entity Relationship Model) according to if only binary relationships are allowed. A binary relationship is a relationship between two entities. Thus, in a GERM, relationships between three or more entities are also ''allowed.''

SEE ALSO