Data Models

Data Model

A data model is a relatively simple representation, usually graphical, of more complex real-world data structures.

Data Models Basic Building Blocks

o   An entity represents a particular type of object in the real world. Each entity occurrence is unique and distinct

o   An attribute is a characteristic of an entity.

o   A relationship describes an association among entities.

§  One-to-many (1:M or 1..*) relationship. A painter paints many different paintings, but each one of them is painted by only one painter..

§  Many-to-many (M:N or *..*) relationship. A student may learn many subjects, and each subject may be learned by many students

§  One-to-one (1:1 or 1..1) relationship. A company’s management structure may require that each of its stores be managed by a single employee. In turn, each store manager, who is an employee, manages only a single store.

o   A constraint is a restriction placed on the data.

Types of Data Models

o   File System Data Model

o   Hierarchical Data Model

Each parent can have many children, but each child has only one parent.

o   Network Data Model

The network model allows a record to have more than one parent

The Network model was created to improve database performance through standards. There are standard specifications for database creation and manipulation for three crucial database components.

1.       The schema

 It is the conceptual organization of the entire database as viewed by the database administrator. The schema includes a definition of the database name, the record type for each record, and the components that make up those records.

2.       The subschema

It defines the portion of the database “seen” by the application programs that actually produce the desired information from the data contained within the database. The existence of subschema definitions allows all application programs to simply invoke the subschema required to access the appropriate database file(s).

3.       A data management language (DML)

That defines the environment in which data can be managed.

To produce the desired standardization for each of the three components, there are three distinct DML components:

·         A schema data definition language (DDL),

o   which enables the database administrator to define the schema components.

·         A subschema DDL,

o   which allows the application programs to define the database components that will be used by the application.

·         A data manipulation language

It is to work with the data in the database. For example SQL

o   Relational Model

A relational table stores a collection of related entities. In this respect, the relational database table resembles a file.

But there is one crucial difference between a table and a file: a table yields complete data and structural independence (exists when it is possible to make changes in the file structure without affecting the application program’s ability to access the data.) because it is a purely logical structure. How the data are physically stored in the database is of no concern to the user or the designer; the perception is what counts.

o   Entity Relationship Model

It is the graphical representation of entities and their relationships in a database structure that complemented the relational data model concepts. The relational data model and ERM combined to provide the foundation for tightly structured database design. ER models are normally represented in an entity relationship diagram (ERD).

o   Semantic Data Model

§  Object oriented Model

Object-oriented data models are typically depicted using Unified Modeling Language (UML) class diagrams.

§  Extended Relational Data Model