See also: ERD
The entity-relation model , or diagram entity-relation or entity relationship diagram in English (shortened in ERD ), is a Data model or Diagramme for high level descriptions of model conceptual of data. It provides a graphic description to represent such data models in the shape of diagrams entity-relation. Such models are used in the phases upstream of design of the computing systems.
They are used, for example, to describe the requirements in Information and/or the type of information which must be recorded in the databases during the phase of Schedule of conditions. The technique of Modélisation of the data can be used to describe all ontology (i.e a total sight and classifications of the terms used and their relations) in a field of interest.
In the case of the design of an information system built on a Database, the conceptual model of data is, at a later stage (usually called logical model), transformed into logical model of data, such as the relational model; then this model is transformed into physical model during the physical phase of design. Sometimes, these two last phases are called " design physique".
This method is employed since the years 1970 to conceive the databases Informatique S.
Principle of the model
At the conceptual level, the entity-relation model distinguishes the objects and the relations:
- the objects of management are for example: an order, a delivery, an invoice,…
- the relations between the objects is for example: an order relates to several products.
The objects are represented by rectangles, the relations by ellipses or rhombuses. The entities, objects or relations, have properties or attributes.
An order can contain several (N) products, and reciprocally the same product can belong to several (N) orders.
A relation (N, N) results in a logical segment. One thus distinguishes the levels conceptual and logical (or organisational).
For the implementation, there is also a physical level, which describe the systems which will be established (central processing unit, database, terminal,…).
The method MERISE, developed about 1975 by the companies Sow-Measured and General Compagnie of Data processing (cgi) largely uses the model entity/relation. The cgi associated data-processing development models.
For more technical details, to also see the page in English on.
In the place of the objects of management, one speaks today about the Objets trade .
Use of the model
The model entity/relation was very employed for the automation of the processes of Gestion in the years 1970 and 1980. It is useful to rationalize the administrative treatments: the Accounts Department, the Pay, invoicing, administration of the sales, purchases, the customer service,…
Gradually all the fields of Gestion were managed by using these models.
In the Years 1990, the majority of the fields of management being already automated, the stake was to replace or adapt management systems become obsolescent. To approximately 60% in the large companies, the specific applications were replaced by integrated business packages.
These software packages preserved the principle of the entity-relation model, in a standardized way: the often incoherent models of a field of management to another had to be harmonized to make function the interfaces between fields.
Data and treatments
The entity-relation model relates primarily to the Donnée S of Gestion.
A Computing system requires to treat Donnée S, there are thus two aspects: Data S and treatments.
The models (Given S and treatments) made it possible to reduce considerably the costs of management by automation of the Tâche S.
The models of treatments used in the Methods of analysis and of design had certain relatively negative effects:
- Process and often managed Task S in a sequential way, the modeling of the treatments appeared sometimes heavy,
- Cloisonnement between the fields of Gestion and the services implied in the Innovation (research and Marketing), from where a difficulty of integrating the processes of management (sequential) and the processes transverse ( quality, Sécurité, Environnement, energy, health), whose Enjeu X become increasingly crucial,
- Tendance to the Financiarisation of the Entreprise S, starting from the large books of General ledger (with the pay and the processes of invoicing).
Integration in a méta-model of town planning
See also: Méta-model of town planning
The entity-relation model was designed for the development of the Logiciel S in the Années 1970, on the basis of rather simple model of Communication used for the Ordinateur S (on the level Computer material).
At that time, the models took very little account of the context (see Communication and model of Claude Shannon, 1948). The only relations between companies (except for rare exceptions) were carried out via the Computing systems Finance companies.
The multiplication of flows of Information of the company with its partners (Extranet), like with its Recipients (Internet, emails), modifies in-depth the Perception of the environment, and reveals transformations deep Sociétale S with the need for adapting the Stratégie S.
If one continues to use the models (given and treatments) as one did in the past, in a sequential way, one is likely to be exceeded compared to the stakes of the current complex projects, been dependant on the Innovation in a world made very open and Interactif by the appearance of technologies Web.
The passage to models of Gestion directed around process linear trades became necessary, in order to return account of the Interaction S multi-trades, multi-rules, and multi-fields of the modern companies.
A more total way of thinking by induction must supplement the analytical way of thinking by Déduction.
In this context, the entity-relation model preserves all its interest to define the Structures of data and the ontology S which are at the base of the Interaction S of the process the ones with the others.
It is a question of integrating the process trade and the Structures of data of the Information system in a Méta-model of town planning (see also Diagrammes of classes UML), to allow the Interaction multi-trades, multi-fields, and multi-rules. The physical processes languages of modeling of the processes trade and the Workflow S must be able to use the data in Orientated architectures services (SOA).
On the data:
- Object trade
- Structure of data
- Relation of several to a
- Relation of one by one
- numerical Identity
On the treatments:
On the urbanization:
- Urbanization (data-processing)
- Méta-model of town planning
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