Relational Database Model

A Relational database model of an application environment is implemented using a general purpose tool available on modern computers which is called a rela­tional database management system. It supports a number of models, each of which has a collection of conceptual objects, actions (operations) on those objects, and constraints under which those actions are performed. Those models coexist in a coherent relational database system which will be viewed as a hierarchy of abstractions. The role of a relational database management system is to implement all those models in a coherent manner, transforming models on higher levels of abstraction into models on lower levels of abstrac­tion all the way down to the machine level.

In a relational database system is viewed through the following four models:

• Global data model of all users.
• Data submodels or views of particular classes of users.
• Structural model of data.
• Multiuser transaction processing model.
• Data distribution model.

In the Relational database model of data, relations are simply unordered sets of tuples. In the structural model, which is one level below the data model in the hierarchy of abstractions, relations have much more complex structure. Within this model relations are equipped with logical orderings of tuples called images. Actions on images are such that they permit decomposition of high-level queries and actions of relational languages into appropriate rela­tional algorithms. While queries and other actions on the level of the data model are nonprocedural and refer to sets of tuples, their representation within the structural model is procedural and contains operations that refer to particular tuples.

A model is a representation of a set of objects and their relationships. Such a representation is always the result of a process of abstraction. During those process relevant objects, relevant relationships among them, and relevant attributes of both objects and relationships are chosen.

The relevance of an object, a relationship, or an attribute is determined by the purpose of the model. Models studied in this book are used for stating questions (queries) about the represented objects and their relationships. Given a set of queries, one can determine to what objects, relationships, and attributes queries from that set refer. Only those associated with the given set of queries will be relevant for the representation within the model.

A model will serve its purpose if it is a clear, correct, up-to-date represen­tation. Attributes of objects and their relationships have specific values which belong to sets called domains. A value of a given attribute may change over time, but it always belongs to the domain of that attribute. Such changes must be reflected in the sets of representations of objects and their relationships.

Questionnaire:

• Explain about Relational database model?