A database is a
collection of related data that have implicit meaning. Database represents
some aspect of the real world, they contain logically coherent data,
and they are designed, built, populated for a specific purpose. The
DBMS architecture is a three-schema architecture, which was proposed
to achieve program data independence, support for multiple views, and
use of catalog to store the database description schema.
The three-schema architecture is a convenient tool for the user to visualize
the schema levels in a database system. Most of the DBMSs do not
separate the three levels completely, but support three-schema architecture.
The three schemas are, only the description of data the only data that
exists in the physical level. What is a schema? The description of database
is called database schema, which is specified during database design
and it is not expected to change frequently. A displayed schema is called
a schema diagram and each object in the schema is called schema construct.
In this DBMS architecture,
schemas can de defined in three levels:
1. Internal level.
2. Conceptual level.
3. External level.
The internal level has an internal schema, which describes the physical
storage structure of the database. The internal schema uses physical
data model, which describes the complete details of data storage, access
paths for the database, and how the datas are retrieved or inserted
in the database. A data model is a collection of conceptual tools for
describing the data, data relationship, data semantics and consistency
The conceptual level has a conceptual schema that describes the whole
database for different users who access the database. The conceptual
schema hides the details of the physical storage structures and concentrates
basically on entities, relationships, and constraints. The external
or view level includes a number of user views. Each external schema
describes the part of the database that a particular user group is interested
in and hides the rest of the database from other user groups. Implementation
data model is used at this level. Each user group will refer to its
own external schema. Hence the DBMS should be capable of transforming
the request specified in the external schema into request against the
conceptual schema. The process of transforming requests and results
between levels are called mappings.
architecture 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. The
three-schema architecture makes it easier to achieve true data independence.
There are two types of data independence, Logical data independence
is the capacity to change the conceptual schema without having to change
the external schemas or application programs. Only the view definition
and the mappings need to be changed in the DBMS that supports logical
data independence. Physical data independence is the capacity to change
the internal schema without having to change the external schemas.
is accomplished because, when the schema is changed at one level the
schema at the next higher-level remains unchanged only the mapping between
the two levels is changed. View is also called as Virtual table
because view does not contain physically stored records and will not
occupy any space. A multi-user database whose users have variety of
applications must provide facilities for defining multiple views. This
three-schema helps us to provide data security of datas among
different users accessing the database, ensures data integrity and avoid
duplication of datas in the database. It helps us to establish
and maintain relationship among the datas in the database.