The Entity Relationships Diagram (ERD)

Résumé du document

The relational model, or the ERD model, was introduced by Dr. Codd in the 70's and has evolved since the first draft which was written by him. The ERD (Entity Relationships Diagram) is a data model or a diagram for high level descriptions of conceptual models of data. It provides a graphic description to represent such data models in the form of diagrams containing the entities and the different associations (or relations) that we have between the elements. Such models are used in the upstream phases of the designing of computing systems. For example, they are used, for example, to describe the information that we need and/or the type of information which must be recorded in the databases during the phase of planning. In the case of the Merise method of the design of information system, the design is based on a database, and the conceptual model of data is, transformed into the logical model of data at a later stage. Like the relational model then, this model is changed into a physical model during the physical phase of design. Sometimes, these two last phases are called "physical design". A basic understanding of the relational model is necessary to effectively use relational database software such as Oracle, Microsoft SQL Server, or even personal database systems such as Access or Fox, which are based on the relational model.

Extraits

[...] We can say that we have a physical and visual representation, which tend to make easier the comprehension of the tool. The third point is the effective communication, and to finish, the integration with the relational database model, which is not a negligible fact. About the disadvantages, we can say that we have a limited constraint representation and a limited relationship representation. In fact, we can miss some relationship and the representation is not a general one. We do this scheme for one problem. The last point is about information. [...]

[...] The next point concerns the screen. The 3 main facts are the accessibility (meaning that if the screen is fitted for blinds and deaf principally), the number of seats and the width of the screen. To finish, the last point is theatre. The main attributes are the name of the theatre, the address and the phone number. Properties of each attributes. After this step and the analyze of the entities and attributes, we have to create the interactions and the links between all of them. [...]

[...] The third main element is called relationships. They are primordial in the diagram, because they are the link between the different elements in the diagram. They are represented by lines between the different entities. The lines made for the relationship indicate that each instance of an entity may have a relationship with instances of the connected entity, and vice versa. The last thing is the Optionality and Cardinality. These symbols, which are at the end of the relationship lines, indicate the optionality and/or the cardinality of each relationship. [...]

[...] In some cases, employee would be an entity, in some it would be an attribute. Once entities, attributes and relationships have been identified and well defined in the first step, the second step can begin. As a consequence, the first draft of the entity relationship diagram can be created Advantages and disadvantages of the ERD Advantages of the ER model can be defined and we can say some of them in this case. In fact, this tool, this model is really simple to use. [...]

[...] The Entity Relationships Diagram (ERD) Executive summary Introduction 1. Theory and explanations of the Entity Relationships Diagram 1.1 Understanding the different tools 1.2 The creation of the ERD 3. Advantages and disadvantages of the ERD 2. Implementation of the ER model Conclusion Bibliography Introduction The relational model, or the ERD model, was introduced by Dr. Codd in the 70's and has evolved since the first draft which was written by him. The ERD (Entity Relationships Diagram) is a data model or a diagram for high level descriptions of conceptual models of data. [...]