1:
Enterprise Data Modal:
During the information system
planning phase.
2: External
Schema / User View:
During the analysis and logical
designing phase.
3:
Conceptual Schema:
During the
analysis phase.
4: Logical
Schema:
During logical
design phase.
5: Physical
Schema:
During the
physical design phase.
It is
important to keep in mind that all these schemas are just different ways of
visualizing structure of the same database by different stake holders.
The three schema architecture as defined by ANSI are as
follow.
1: External
Schema:
This is a view of managers and
other employer, who are the database users. The External Schema can be
represented as a combination of the enterprise Data Model and a collection of
detailed user-views.
2:
Conceptual Schema:
This schema
combines the different external views into a single and comprehensive
definition of the enterprises data. The conceptual schema represents the view
of the data architecture or data administrator.
3: Internal Schema:
Internal
Schema consists of two separate schemas.
i.
Logical Schema
ii.
Physical Schema
I: Logical
Schema:
The logical
schema is the representation of data for a type of data-management technology.
II:
Physical Schema:
Describes the
how data are to be represented and stored in secondary storage using a
particular DBMS.