Data modelling

Data modelling

Data modelling

When a relational database is to be designed, an entity-relationship diagram is drawn at an early stage and developed as the requirements of the database and its processing become better understood. Drawing an entity-relationship diagram aids understanding of an organization's data needs and can serve as a schema diagram for the required system's database. A schema disgram is any diagram that attempts to show the structure of tha data in a database. Nearly all systems analysis and design methodologies contain entity-relationship diagramming as an important part of the methodology and nearly all CASE (Computer Aided Software Engineering) tools contain the facility for drawing entity-relationship diagrams. An entity-relationship diagram could serve as the basis for the design of the files in a conventional file-based system as well as for a schema diagram in a database system. (Date, C. J. 2003, 65-78)

In an entity-relationship diagram an entity type is shown as a box. In given Fig. CUSTOMER is an entity type. Each entity type is shown once. There may be many entity types in an entity-relationship diagram. The name of an entity type is singular since it represents a type. An entity type is considered to be a set of objects. For this reason some people use the alternative term entity set. An entity is simply one member or example or element or instance of the type or set. So an entity is one individual within an entity type. For example, within the entity type CUSTOMER, J. Smith might be one entity. He is an individual entity within the type, an element in the set, an instance of the type 'customer'

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There will be the same number of attributes for each entity within an entity type. That is one of the characteristics of entity-relationship modelling and relational databases. We store the same type of facts (attributes) about every entity within the entity type. If you knew that one of your customers happened to be your cousin, there would be no attribute to store that fact in, unless you wanted to have a 'cousin-yes-no' attribute, in which case nearly every customer would be a `no', which would be considered a waste of space. (Date, C. J. 2003, 65-78)

A relationship type can be classified by the number of entity types involved, and by the degree of the relationship type, as is shown in given Fig... These methods of classifying relationship types are complementary. To describe a relationship type adequately, you need to say what the name of the relationship type and its inverse are and their meaning, if not clear from their names and you also need to declare the entity type or types involved and the degree of the relationship type that links the entities. We now discuss the latter two items. The purpose of discussing the number of entity types is to introduce the terms unary relationship type, binary relationship type, and ternary relationship type, and to give examples of each. The number of entity types in the relationship ...