Relationships between elements of data models
Establishing relationships between database model elements is one of the most important tasks in database modeling, since it determines not only the fact of interaction of individual structural elements among themselves, but also sets rules for solving a number of additional tasks:
- determination of the number of connected instances for each structural element;
- determination of the possibility of storing empty values in the fields by which communication is performed;
establishing referential integrity rules that determine the characteristics of performing actions on the data of structural element instances;
highlight key attributes in the structure elements of the database;
- other tasks that ensure efficient work with data.
A link in a database model is an indication of the interaction of two attributes of different structural elements represented by keys.
Depending on the level of representation of the database model (conceptual (object), logical, physical, etc.), communication can determine the appropriate meaning of the interaction of structural elements. So, for the conceptual level, when the developer has not yet completed the domain analysis, but has already started modeling the database, the connection between the structural elements (objects) denotes the information interaction of the instances of the two linked objects, constituting an intermediate information-object model of the database.
The information-object model is understood as a diagram of the interrelationships of objects in the domain with the selection of the attributive composition of objects, the semantic content of the connection in the essence of activity in the domain and the number of interacting instances.
To build this model, Peter Chen's notation can be used, oriented to describing the database model at the level of functional structural elements (objects) that must be represented in the database model by logical entities and, subsequently, to be converted into database tables. When it comes to the logical level of representation of a database model, a much larger number of structural elements (entities) are used here, some of which may not appear explicitly in the domain and appear only in the selection stages of the auxiliary structural elements. These database models are represented by different notations of logical and mythological modeling (for example, SHER 1x).
If an object-oriented information system is being developed, then the database model must be associated with the objects of the future software application and often it is included in the object classes. For such models, UML notation, originally oriented to work with object structures, is applicable. In this case, the essence and meaning of the links between the structural elements remain identical, regardless of the selected notation of the representation of the database model. In modeling, four basic types of links are distinguished, which determine the entire aggregate of possible interactions of structural elements: one-to-one, one-to-many, many-to-many, categorization.
When determining the type of communication, the developer must answer two questions: "How many instances of the first structural element can be associated with one instance of the second structural element?" and "How many instances of the second structural element can be associated with one instance of the first structural element?".
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