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 existence.
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
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