An attribute of an entity set is a function that maps from the entity set into a domain. Since an entity set may have several attributes, each entity can be described by a set of (attribute, data value) pairs, one pair for each attribute of the entity set.
An attribute, as used in the E-R model, can be characterised by the following attribute types.
- Simple / Composite
- Simple attributes are not divided into subparts (i.e., other attributes).
- Composite attributes can be divided into subparts.
- Single-valued / Multivalued
- Single-valued attributes have a single value for a particular entity.
- Multivalued attributes have a set of values for a specific entity.
- Derived
- The value of derived attributes can be derived from the values of other related attributes or entities.
An attribute takes a null value when an entity does not have a value for it.