The conceptual data schema is a tool of the Merise method, a method developed in the 1970s in the context of computerization of business systems. The conceptual diagram is an analytical step in the development process of a database construction type IT project.
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What is a conceptual diagram?
The conceptual diagram models the problem to be dealt with when building an information system (IS). Concretely, the conceptual data diagram, also called MCD for conceptual data model, is a clear representation of the data of the information system to be designed. This representation also shows the relationships between these data.
The conceptual data schema, as a schematic representation, is easily understandable. Readable by the layman, the CDM does not use computer language. Under these conditions, it constitutes a valuable tool during the preliminary study of an IT project. Stakeholders, whatever their level of technical skills, are in fact able to communicate without difficulty; information is shared within the project team in an unequivocal manner. And insofar as each stakeholder understands the conceptual diagram, the project bases are solid: each one with his expertise can verify that the problem is correctly addressed, each one adds any details useful to the success of the project.
At the stage of the conceptual data schema, it is a question of analyzing the invariants on the basis of which the application is to be created. The invariants are analyzed independently of the subsequent technical choices necessary to design the application.
How does a concept map work?
The conceptual diagram shows several concepts, distinct but interdependent blocks of data:
- Data blocks are represented in rectangles.
- The rectangles are linked together by arrow lines, indicating the nature of the functional dependency relationship that exists between the data blocks.
Example: the company wants to design an information system to manage the issuance of its customer invoices. The diagram shows the following elements:
- A rectangular “INVOICE” data block.
- A rectangular “CUSTOMERS” data block.
- An arrow line with the words “Emission”.
Here, the conceptual data diagram represents invoices issued to customers. The person in charge of the design of the IS thus knows which data must be integrated, and what is the relationship between these data.
The conceptual diagram is very useful when the problem to be processed by computer integrates many data blocks and complex dependency relationships. The CDM indeed models a global view: the problem is easier to grasp, and the risk of omissions or errors is limited when designing the solution.
Once ready, the conceptual diagram is used as a basis for technical choices on the one hand, organizational on the other hand: how to build the information system and which team to dedicate to the IS project? At its end, the methodological process leads to a concrete result.
How to create a conceptual data diagram?
The MCD is built on the basis of 2 central elements: entities and associations. This is why it is common to speak of an entity / association model. To create a conceptual data diagram, 5 steps are required.
1 – List the entities
Entities are the data blocks that must be integrated into the IS. Each entity includes physical objects or abstract concepts, which can be grouped together because they have comparable characteristics. Conventionally, entities are shown in rectangles, and the name of each entity is in upper case.
Example of entities: to process a room reservation problem on a computer, the diagram represents the entities: “ROOMS” and “CUSTOMERS”.
2 – Determine the attributes and specify an identifier
2nd step to draw the conceptual diagram: for each entity, you must specify the attributes and choose an identifier.
- Attributes are the characteristics of the entity.
- The identifier is unique. Conventionally, the identifier is the 1st of the list of attributes, it is underlined.
In the previous example:
- The attributes of the “ROOMS” entity: room number, surface area, number of beds, presence or absence of balcony, bath or shower. The identifier of the “ROOMS” entity is the room number, because each room has a unique number.
- The attributes of the “CUSTOMERS” entity: surname and first name, date of birth, e-mail address, telephone number. The identifier of the “CUSTOMERS” entity is the e-mail address attribute because an e-mail address is unique, it makes it possible to identify a customer.
3 – Materialize associations between entities
This step involves drawing on the conceptual diagram the arrow lines between the entities to be associated. These lines represent the links between the data blocks. Conventionally, the nature of the relation is specified in an ellipse or in a rhombus. Note that like the entity, the relation can have attributes.
In the previous example: customers are responsible for reserving rooms. The 2 entities are therefore linked to each other, and the association bears the title “Reserve”.
4 – Note the cardinalities
A cardinality, in the conceptual data schema, represents the number of times the entity is involved in the relationship. An entity can be:
- Never Involved: Cardinality is zero.
- Involved only once: the cardinality is equal to 1.
- Involved more than once: the cardinality is equal to n.
Next to each entity, note the minimum cardinality and the maximum cardinality.
In the previous example:
- The “ROOMS” entity: a room can be reserved or not. The word “0.1” is noted next to the entity.
- The “CUSTOMERS” entity: the customer reserves at least once, failing which he is not registered as a customer in the database. The customer can book multiple rooms. The words “1, N” are noted next to the entity.
5 – Check and enrich the conceptual diagram
Once the diagram is finalized, the stakeholders in the IS project are able to understand it, to check its consistency and possibly modify or enrich it. When the conceptual data diagram is ready, the project team implements the following system design steps.
Illustration of a conceptual diagram of data for the computerized management of a company’s product sales cycle:
- The entities: as part of the sales cycle, the company integrates data relating to suppliers, products and customers. The entities are represented by 3 rectangles: “SUPPLIERS”, “PRODUCTS”, “CUSTOMERS”.
- The attributes: the “SUPPLIERS” entity carries the attributes n °, company name and e-mail, it is identified by the supplier n °; the “PRODUCTS” entity carries the attributes reference, nature and price, it is identified by the reference; the “CUSTOMERS” entity has the attributes number, name and e-mail, it is identified by the customer number.
- Associations: the supplier delivers the product when the customer orders it. There is therefore a link between the supplier and the product, materialized by a line entitled “Deliver”, and a link between the customer and the product, materialized by a line entitled “Order”.
- Cardinalities: the supplier delivers at least 1 product, otherwise it is not registered as a supplier in the database; the supplier can deliver several products; note “1, N” next to the entity “SUPPLIERS”. The product, likewise, is delivered 1 or more times: note “1, N” next to the entity “PRODUCTS”. The customer orders from 1 to several times: note “1, N” next to the corresponding entity. On the other hand, a product may not be ordered, or may be ordered several times: note “0, N” next to the entity.
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