11.2. Hierarchical, network and relational data presentation models

The information in the database is in some way structured, that is, it can be described by the data representation model (data model) that are supported by the DBMS. These models are divided into hierarchical, network and relational.

When using a hierarchical data representation model , relationships between data can be characterized using an ordered graph (or tree). In programming, the tree type is used to describe the structure of a hierarchical database.

The main advantages of the hierarchical data model are:

1) effective use of computer memory;

2) high speed of performing basic operations on data;

3) convenience of working with hierarchically ordered information.

The disadvantages of the hierarchical data presentation model include:

1) the bulkiness of such a model for processing information with rather complex logical connections;

2) the difficulty in understanding its functioning as an ordinary user.

A small number of DBMS built on a hierarchical data model.

The network model can be represented as the development and generalization of a hierarchical data model that allows you to display a variety of data relationships in the form of an arbitrary graph.

The advantages of the network model of data presentation are:

1) efficiency in the use of computer memory;

2) high speed of performing basic operations on data;

3) enormous opportunities (greater than the hierarchical model) the formation of arbitrary connections.

The disadvantages of the network model of data presentation include:

1) high complexity and rigidity of the database schema, which is based on it;

2) difficulty in understanding and performing information processing in a database by a non-professional user.

Database management systems based on the network model are also not widely used in practice.

The relational model of data representation was developed by an employee of the company 1VME. Codd His model is based on the concept of "relationship" (relation). The simplest example of a relationship is a two-dimensional table.

The advantages of the relational model of data presentation (as compared to the hierarchical and network models) are its comprehensibility, simplicity and convenience of practical implementation of relational databases on computers.

The disadvantages of the relational model of data representation include:

1) the lack of standard means of identifying individual records;

2) the complexity of describing hierarchical and network connections.

Most of the DBMS, used by both professional and non-professional users, are based on the relational data model (Visual FoxPro and Access from Microsoft, Oracle from Oracle , etc.).