Data Models

Data Model
Is a relatively simple representation, usually graphical, of more abstraction of a more complex real- world object event, a data model represents data structures and their characteristics , relations , constraints , transformations , and , other constructs with purpose of supporting a specific problem domain .


The Importance of Data Models
Data models can facilitate interaction among the designer, the applications programmer, and the end user, a well- developed data model can even faster improved understanding of the organization for which the database design is developed.


Data Model Basic Building Blokes

Entity: is anything (a person, a please, a thing, or an event) about which data are to be collected and stored. An entity represents a particular type of object in the real world entities are "distinguishable" that is each entity occurrence is unique and distinct. For example, a CUSTOMER entity would have many distinguishable customer occurrences such as Ahmed, Ali, etc.

Attribute: is a characteristic of an entity. For example, a CUSTOMER entity would bed described by a attributes such as customer last name, customer first name, its equivalent of fields in the file systems

Relationship: describes an association among entities for example a relationship exists between customers and agents that can be described as follows: an agent can serve many customers, and each customer may be served by one agent.

Data models use three types of relationships :
• one-to-many(1:M or 1..*)relationships
A painter paints many different paintings, but each one of them is painted by only one painter.
• Many-to-many (M: N or *..*) relationship
An employee may learn many job skills many be learned by many.
• one-to-one( 1:1 OR 1..1) relationship
A retail 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, managed only a single store.

Constraint: is a restriction placed on the data. Constraints are important because they help to ensure data integrity. And it s normally expressed in the form of rules.
• An employee s salary must have values that are between 6000 and
350000
• Each class must have one and only one teacher.