Data Model & Classification of Data Models

DEFINITION:

A data model is an abstraction of reality. The reality may be a business system or an engineering system.

DATA MODEL:

Data model is an abstraction of data about entities of a system .The structure of the data base system is the concept of data model .Thus, it is a collection of conceptual tools for describing data, data relationships, data semantics & constraints. Therefore, a data model is relatively simple representation within the database environment.

For the conceptual data model, We define an entity relationship model which is known as   ER-Diagram. It is a special form of network data model in which we use  certain symbols to indicate the name of the associations between the meaningful pairs of relations in the conceptual model.

The importance of ER-DIAGRAM is that, they help the database designer to have a better insight of the model. This will in turn, help the database designer to implement the conceptual data model into any one of the following data models.

1.   HIERARCHIAL DATA MODEL (HDM)
2.   NETWORK DATA MODEL (NDM)
3.   RELATIONAL DATA MODEL (RDM)

We use ellipses to indicate the fields of a relational table and use a rectangular box to specify the table and its relationship around the relation. 

The associations between the fields of a file can be categorized into,

A) one-to-one association(1:1<->)

B) one-to-one association(1:M<->)

C) Many- to Many association(M:M<->)

D) Many-to –one association(M:1<->)

E) one-to-one conditional association(<-0->)

CLASSIFICATIONS OF DATA MODELS:

The data models are classified into 4 types

1)  External View or Users view

2)  Conceptual Data Model

3)  Internal Data Model

4)  Physical Data Model

The arrangement of all the files for an entire organization is called schema. The schema defines a schematic representation or over all view of all the files in a database. A portion of the schema of a database is called sub- schema.

In database environment, the data models are discussed on the basis of their degree of abstraction.

1. EXTERNAL VIEW OR USER VIEW

The different sub-schemas of the data base is called as user view or external view. For example, a production, planning, stores department, quality control, maintenance schedule etc, are sub schemas of a production system. Similarly funds flow analysis, breakeven analysis, costing, payroll, materials account etc are the sub-systems of a Finance System.

2. CONCEPTUAL DATA MODEL

It is a overall or consolidated view of all the user views. It gives a comprehensive view of the entire data base. The conceptual data model that represents global view of the data. This model is the basis for identification & description of main data avoiding details. The most widely used conceptual data model in database environment is, ER-model. In conceptual data model, the details either in hardware or software will have no effect on the data base design at the conceptual level.

3. INTERNAL DATA MODEL

After planning the conceptual data model, it is mapped or implemented into any one of the data models such as HDM, NDM or RDM. Then, it is called as Internal Data Model.

The internal data model requires the database designer to match the conceptual data model characteristics & the other constraints to the above 3 data models. Thus, the internal data model depends on the existence of a specific database software.

4. PHYSICAL DATA MODEL   

The physical data model operates at the lowest level of abstraction, describing the way the data stored or saved  on the storage media such as hard disc, magnetic tapes etc.

The physical data model is derived from the internal data model after incorporating certain methods and defining all storage specifications.