Extended ER model
DEFINITION
The extended ER model (E entity-relationship) model is a language for defnition of structuring (and functionality) of database or information systems. It uses inductive development of structuring. Basic attributes are assigned to base data types. Complex attributes can be constructed by applying constructors such as tuple, list or set constructors to attributes that have already been constructed. Entity types conceptualise structuring of things of reality through attributes. Cluster types allow the generalise or to combine types into singleton types. Relationship types associate types that have already been constructed into an association type. The types may be restricted by integrity constraints and by speciØcation of identiØcation of objects deØned on the corresponding type. Typical integrity constraint of the extended entity-relationship model are participation, lookup and general cardinality constraints. Entity, cluster and relationship classes contain a Ønite set of objects deØned on these types. The types of an EER schema are typically depicted by an EER diagram.
IMPORTANT POINTS
The extended entity-relationship model uses a data type system for its attribute types, allows to construct entity types E$(attr(E);ßE) through a set of attributes, and inductively builds relationship types
R$(T1;:::;Tn;attr(R);ßR) of order I (I ∏ 1) through a set of (labelled) entity types and of relationship types of order less than I and a set of attribute types. Additionally, cluster types
C $ T1 : [::::[Tn of order I can be deØned through a disjoint union :[of relationship types of order less than I or of entity types. Entity/relationship/cluster classes contain a set of tuples of the entity/relationship/cluster types. The data type system is typically inductively constructed on a base type B system by application of constructors such as the tuple constructor ( :: ), set constructor f :: g and the list constructor < :: >. Types may be optional [ :: ]. The types T can be labelled l : T. The types T are typically extended by a set ßT of constraints which are deØned for components of the
type. Only those classes are considered for which the constraints of their types are valid. An entity-relationship schema consists of a set of data, entity, relationship and cluster types which types are inductively built on the basis of the base types.
Given a base type B system. The types of the ER schema are deØned through the type equation:
T = B j ( l 1 : T;:::;l n : T ) j f T g j <T > j [T] j T : [ Tj l : Tj N $ T
A set of entity, cluster and relationship types can be represented by ER diagrams. The diagram representation of EER schemata uses rectangles and diamonds for the entity and relationship types. Attribute types are associated with the corresponding type
