Database Reference
In-Depth Information
CONCEPT
Amount
IDENTITY
DOUBLE amount
CHAR(3) currency
ENTITY
// Empty
If both constituents of a concept are non-empty then identity class describes references for representing
its objects. For example, let us assume that we need to model bank accounts. In this problem domain any
bank account is identified by an account number and characterized by its current balance and owner. The
existence of domain-specific identity and entity is easily described by means of the following concept:
CONCEPT
Account
IDENTITY
CHAR(10) accNo
ENTITY
Amount balance
Person owner
Each instance of this concept will be a pair of account reference identifying this element and account
object consisting of two fields. Variables of this concept will store account numbers which identify the
corresponding account objects. Account owners can be modeled using the following concept:
CONCEPT
Person
IDENTITY
CHAR(11) ssn
ENTITY
CHAR(11) name
Date dob
The difference from the relational model (Codd, 1970) is that concept dimensions contain the whole
identity of the referenced element treated as one value. The difference from object data models (Dit-
trich, 1986; Bancilhon, 1996) is that identities may have arbitrary domain-specific structure. And the
general specific feature of this approach is that both identities and entities are modeled together within
one data modeling construct.
MODELING HIERARCHIES
Any element must have some identity which manifests the fact of its existence. But if something exists
then there has to be some space to which it belongs, that is, elements are not able to exist outside of
any space. In COM, existence within space means that the element is identified relative to this space.
Space is a normal element of the model and all elements exist within a hierarchy where a child is said
to be
included
in its parent interpreted as a space, scope, context or domain. Parents in the inclusion
