0.2 Introduction to algorithms  (Page 5/8)

Data Model Theory

A data model theory has three main components:

• The structural part: a collection of data structures which are used to create databases representing the entities or objects modeled by the database.
• The integrity part: a collection of rules governing the constraints placed on these data structures to ensure structural integrity.
• The manipulation part: a collection of operators which can be applied to the data structures, to update and query the data contained in the database.

For example, in the relational model , the structural part is based on a modified concept of the mathematical relation ; the integrity part is expressed in first-order logic and the manipulation part is expressed using the relational algebra , tuple calculus and domain calculus .

Data Model Instance

Data modeling is the process of creating a data model instance by applying a data model theory. This is typically done to solve some business enterprise requirement.

Business requirements are normally captured by a semantic logical data model . This is transformed into a physical data model instance from which is generated a physical database . For more information on the tools and techniques of data modeling, see data modeling .

For example, a data modeler may use a data modeling tool to create an ERD of the Corporate data repository of some business enterprise. This model is transformed into a relational model , which in turn generates a relational database .

Pic.1 Zachman Framework Perspectives of Data Focus

A data model instance may be one of three kinds (according to ANSI in 1975):

• a conceptual schema (data model) describes the semantics of an organization. This consists of entity classes (representing things of significance to the organization) and relationships (assertions about associations between pairs of entity classes).
• a logical schema (data model) describes the semantics, as represented by a particular data manipulation technology. This consists of descriptions of tables and columns, object oriented classes, and XML tags, among other things.
• a physical schema (data model) describes the physical means by which data are stored. This is concerned with partitions, CPUs, tablespaces, and the like.

The significance of this approach, according to ANSI, is that it allows the three perspectives to be relatively independent of each other. Storage technology can change without affecting either the logical or the conceptual model. The table/column structure can change without (necessarily) affecting the conceptual model. In each case, of course, the structures must remain consistent with the other model. The table/column structure may be different from a direct translation of the entity classes and attributes, but it must ultimately carry out the objectives of the conceptual entity class structure. Early phases of many software development projects emphasize the design of a conceptual data model . Such a design can be detailed into a logical data model . In later stages, this model may be translated into physical data model .