Entity–relationship model: Difference between revisions

Revision as of 15:25, 15 June 2006

Revision as of 09:44, 23 June 2006

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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 further described as multi-valued, composite, or derived. A multi-valued attribute, illustrated with a double-line ellipse, may have more than one value for at least one instance of its entity. For example, a piece of software (entity=application) may have the multivalued attribute "platform" because at least one instance of that entity runs on more than one operating system. A composite attribute may itself contain two or more attributes and is indicated as having at least contributing attributes of its own. For example, addresses ususallyusually are composite attributes, composed of attributes such as street address, city, and so forth. Derived attributes are attributes whose value is entirely dependent on another attribute and are indicated by dashed ellipses. For example, if we have an employee database with an employee entity along with an age attribute, the age attribute would be derived from a birth date attribute.

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 (computer science)|superclass]] is connected to the point on top and the two (or more) [[subclass (computer science)|subclass]]es are connected to the base.