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Merise         
a SHORT introduction            

Summary

Many projects use a data base: simple Internet site calling a MySQL data base, an application calling using Oracle for example.
To have a data base correctly thought, ordered, is thus quite as important as the development of the project being useful of the aforesaid the data base.

The Merise method represents a method of design of information system and has several levels: conceptual, organizational, logical, physical.
The conceptual level is the level relating to the modeling of a data base leading in particular to a conceptual model of data (known as MCD), which is in some kinds a diagram of the data base.
The other levels suggested by Merise will be briefly presented

1.1 Merise

Merise is a method of design, development and realization of data-processing projects. The goal of this method is to manage to conceive an information system. The Merise method is based on separation of the data and the treatments to be carried out in several conceptual and physical models.

The Merise method goes back to 1978-1979, and made following a national consultation launched in 1977 by the ministry of  Industry with an aim of choosing companies of data processing consultant in order to define a method of design of information systems. The two principal companies having developed this method are the CTI (Technical Center of Data processing) charged to manage the project, and the CETE (Center of Technical Studies of the Equipment) established in Aix-en-Provence.

Merise breaks up an information system into levels going of the abstract towards the concrete one:

1.2 The conceptual level

It is on this level that one schematizes the data of the information system.

Two models are used on this level:
- the conceptual model of data (MCD) which is a description of the data and relations, it is this model which is used to model a data base
- the conceptual model of treatments (MCT) is the description of the dynamic part of the information system and described the actions to be carried out on the data, and expresses the translation of the rules of management composing the activity of the company

1.3 The organisational level

The choices of organization are taken into account on this level:
- the distribution of the treatments between the man and the machine
- operating mode (real or differed time)
- the assignment of the data and the treatments

Two models are associated this level:
- the organizational model of data (MOD) takes again the formalism used in the MCD, but which takes account of other constraints (rights of access, site, etc)
- the organizational model of the treatments (WORD) represents by procedure the phases and the tasks carried out by each working station

1.4 Logical and physical levels

- The logical level
For the treatments, the logical level makes it possible to describe the technical design which treats mainly structuring in treatment units of real or differed the time type.
For the data, the logical level makes it possible to take into account the technical structuring suitable for computerized storage.

- The physical level
At this level, the choices of the technical tools are defined.

2. The conceptual model of data (MCD)

The MCD is the model of the Merise method making it possible to obtain a diagram of a data base.
To carry out a good MCD makes it possible to be ensured of the quality of a data base: logic, redundancy, etc

2.1 Basic concepts

- The concept of entity
An entity is an object equipped with a clean existence.



Thus for example one will be able to find an entity “Customer” in whom one will find all the customers of the base.

- The concept of relation
A relation between entities is an association perceived in reality between two or several entities.

- The concept of properties
A property is a data item which one has on an entity or a relation

- The concept of cardinalities
The cardinalities of an association make it possible to specify the number of occurrence of the object.
One indicates in first the occurrence minimal and second the maximum occurrence.
“0, N” mean for example that the relation can not exist.
“1,1” means that the relation exists only with only one another entity.

2.2 Complements on the entities and the relations

- Identifiers
It is a particular property of the entity such as to each value of the property corresponds one and only one occurrence of the entity

A relation has by defect like identifier the concatenation of the identifiers of the entities taking part in the relation.

- Dimension of a relation
It is the number of entities taking part in the relation.
Between two entities it is a binary relation, between three entities it is a ternary relation, between N relation, it is a n-ary relation.

- Reflexive Relation
It is a relation of entity on itself.

2.3 Advanced concepts

- Constraint of functional integrity (CIF)
A CIF represents a strong and permanent bond of dependence of an entity compared to another.

- The concept of heritage
The heritage is a process which consists in transmitting the properties of the standard entity towards the sub-types

- Constraints of extension on the relations or entities
What was evoked previously does not make it possible to manage certain constraints and characteristics:
  . constraint of inclusion: connect two relations and expresses the fact that the entities taking part in relation 1 take part implicitly in relation 2
  . constraint of exclusion: connect two relations and expresses the fact that the entities taking part in relation 1 do not take part in relation 2
  . constraint of totality: connect two relations and expresses the fact that the entities taking part in the relations must take part at all costs in the one of both.

3. Other models

3.1 The conceptual model of treatments (MCT)

The MCT is a diagrammatic representation of the activity or a subset of activity of a company. In other words, the MCT makes it possible to represent the actions carried out by the company for the realization of its finalities.

3.2 The organisational model of the treatments (MOT)

The MCT made it possible to break up a process in operations describing the activity of the company thus.
That must be supplemented by the catch in consideration of the organization chosen by the company.

The MOT  makes it possible to represent the whole of the treatments by taking of account the organization of the company, materialized by working stations.

Conclusion

This article made it possible to propose the creation of a conceptual model of data in order to correctly conceive a data base thanks to Merise.

The other aspects of Merise were also approached, Merise not being limited only to the modeling of a data base.

Like love or peace or freedom, you never get there once and for all. You just have to keep going toward them. In these exhibit cases you can see the steps we tried to take in those directions.