Three level architecture
Database Task group developed and published a proposal for a standard vocabulary
and architecture for database system in 1971. It was appointed by a conference
on database system and Languages. The Standards Planning and requirement
committee of American National standards institute committee on information processing
developed and published a similar vocabulary and architecture in 1975.
The result of these reports was the three level architecture. The three level architecture
is the basic of modern database architecture. Database can be viewed at three levels. The three
levels are depicted by three models known as three level schema. The models refers to the structure
of the database not the data stored on it. The parmanent structure of database is known as
intension of database or schema. The data stored at a given time is known as extension of database
or database instance.
The Intension of a database should not be changed once it has been defined. This is because a small
change in the intension of database may requires many changes to the data stored in database
. The extension of database is performed according to the rules defined in the intension of database.
The schema is used to store definition of the structure of the database. It can be any thing
like a single intity or the whole organization. Three level architecture define different
schemes are stored to the different levels to isolate the details of different levels from
one another.
external level
The external level consists of different external views of a database. Each external
view is the view of a particular user about the system. Different users conceive
the system in the different ways. Each users is interested in one part of the system and
ignores other parts. One user may not be aware of the whole system at all.
Different views may have different representations of the same data. For example,one
user may think that dates are stored in form. Another user may think they are represented
as year/month/day.
Some views might include virtual or calculated data. Virtual data is the data that is not actually
stored in database but created when needed. For Example, the product name and its
price can be used to stored in database. The total bill can be created as and when desired
so Similarly can be calculated when result card is prepared.
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