At the “downstream” design structural design is carried out from top to bottom. Such a design is called analysis – there is a whole study (description of the subject area), then the division of the whole into its component parts and then be consistent study of these parts.
Stages of database design method of “top-down” design shown in Figure 1.
Figure 1 – Steps of designing a database by ” top-down ” design
To display method uses the following symbols: circles described in the names of the stages of design, in rectangles – results[1-3]. Design begins with an analysis of the subject area and describe the formation of an external database level, which brings together represent all users to develop database to identify classes of objects (entities) subject area, links between them. On the basis of the description of the external level conceptual information is based logical domain model (BLM), and then based on it get datalohic model (DLM) database. DLM is the basis for the next stage of the design database – phase formation of the physical database model. Such an approach to the design of the database is also called a conceptual or conceptual design.
The conceptual approach to the design of the database are the following three areas:
-Real world or object system;
The main components of the system are the object: the object (entity instance), property (attribute), the ratio of (communication) (defined subject area, if known existing objects, their properties and relationships between them)
The object in the conceptual approach – this is what the information system should collect information.
Objects grouped into classes of objects (instances of entities in nature, or they say – the type of entity). An object class may consist of one or more objects. For example, the object class individual, the individual objects – Ivanov, Petrov, Sidorov.
Each object class must have a unique identifier that uniquely identifies each individual object (entity instance) in the class of objects. Each object class should have some properties (attributes), the number of which is the same for each object in the object class, the value of each property can be different in different sites. Each feature class can have any number of links with other classes of objects.
Based on top-down approach is the idea of establishing a consistent matching between object system, information and further datalohic areas. Is a sequential transformation of understanding of domain objects and relations between them in a formal description logic information of the subject area and further conversion of the logic information of the subject area the description of the structure of the database in terms of selected data structures – construction logic and data. This gradual transformation allows a clear and simple manner to realize correct displaying of the sense of the real world in a database.
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