Why there is a need for relationship among the datas in the database system?
A relational database
is a shared repository of data. A relationship is defined, as an association
of entities, while relationship instance is an association of entity instances.
Entity is defined as a thing in the real world with independence
An entity may be an object with physical existence For example, car, house, person or it may be an object with conceptual existence Example University, company. A relationship may associate an entity with itself. Several relationships may exist between same entities. They are different types of relationship among various data stored in the databases are, (i) one to one (ii) one to many or many to one (iii) many to many.
Each entity has attributes the particular properties that describe it. A database normally contains groups of entities that are similar. The collection of all entities of particular entity type in the database at any point in time is called entity set. Entity plays key role in representing E-R diagram (Entity Relationship diagrams) that is diagrammatic representation of relationship among datas present in the databases.
Relational database can be used to represent complex relationships among datas as well as to retrieve and update related data easily and efficiently. Hence it is possible to develop models for the database design using high-level conceptual datas.
The conceptual design
has a conceptual schema, it is nothing but a concise description of the
data requirements of the users and includes detailed description of entity
of entity types, relationships, constraints, and these are expressed using
the concepts provided by the high-level data model. If any DBMS satisfies
Codd twelve rules and it should satisfy relational theory in algebra then
the database is called relational database management system.
Since it maintains interrelationship among the datas present in the database, data integrity, and provides a multi user environment it is referred as RDMS. To qualify a genuine RDBMS, the system should possess some inherent properties. (a) It must store data as a relation that each column is independently identifies by its column name and the ordering of rows is immaterial. (b) The operations available to he user, as well as those internally used by the system, should be true relational operations, that is they should be capable of generating new relation from the old relations.
Data model is plays a key role in establishing in relationship among the datas, it includes a set of operations to manipulate the data, and the basic set of operations constitutes relational algebra. The result of retrieval is a new relation, which may be formed from one or more relations. This relational algebra thus produces new relations, which can be manipulated using operations of the same algebra. A sequence of relational algebra operations forms a relational algebra expression whose result will also be a relation. It is possible to define and enforce integrity constraints for the datas stored in the database and helps in avoiding unauthorized access to the datas.
If a Relationship
is present among the datas in the database system it is possible
to allow multiple users to access the database at the same time and sharing
of data is possible. It is also easy to retrieve and update information
into database system. Data redundancy is avoided that is storing of datas
multiple times which result in wastage of space, this may result in data
inconsistency in the application. Moreover in database approach multiple
views can be created.
View is also called as Virtual table because view does not contain physically stored records and will not occupy any space. Security is one of the important aspects of RDBMS, the mechanism is used to prevent destructive interaction of different users accessing the unauthorized datas. This mechanism uses locks to prevent unauthorized by different users. The relational database system uses three-schema architecture, which explains the concept of data independence, which is defined as the capacity to the change the schemas at one level of the database system without having to change the schema at next higher level.
Physical and logical structure of the databases should remain unaffected even though any changes are done in physical or in the logical level of the databases. The three-schema architecture makes it easier to achieve true data independence. The structure of the database is stored separately in the system catalog from the access of the application programs.
This property is known as program-data independence. The database also provides facilities for recovering hardware and software failures. The backup recovery subsystem of the RDBMS is responsible for recovery of datas, it also ensures that the program is resumed from the point at which it is interrupted so that its full effect is recorded in the database. It requires only limited time to develop an application.
The different levels in system should not revert or bypass the integrity rules and constraints. The integrity constraints should be stored in the system catalog of the database.