Database Reference
In-Depth Information
External Schema
Conceptual Schema
Physical Schema
USER
VIEW-1
USER
VIEW-2
PHYSICAL
LOGICAL
STRUCTURE
STRUCTURE
USER
VIEW-3
USER
VIEW-n
Figure 4-12
Logical versus physical design.
Figure 4-12 indicates the output from the logical design and physical design
steps. “Schema” means a way of indicating the output of the design phase.
“External schema” constitutes the set of external views of various user groups.
“Conceptual schema” represents the structures of the database as a whole. Finally,
“physical schema” consists of the files and records as stored in hardware. Creation
of the external schema and the conceptual schema are generally reckoned as logical
design activities; whereas production of the physical schema is the physical design
step.
Go back to the example of the process of settling an auto insurance claim.
Imagine that your proposed database just has the data required to perform this
process. We will describe the external, conceptual, and physical schema for the data-
base of an auto insurance company in the next three subsections. For the sake of
keeping it simple, we will deal with only a few data elements.
The External Schema
Try to understand what exactly “external schema” means and why it is noted as
external . Imagine that some user group is looking at the data structures represented
in the database from outside, from a standpoint external to the database. What
would that user group like to see in the database? The group is interested in several
data elements inside the database, most probably not all of it. When that group looks
at the database, for the group the database is a collection of data elements that are
important to it; that set of data elements is the group's view of the database.
The external schema is the collection of such views of different user groups.
Figure 4-13 clarifies the concept further with an example from the database of an
auto insurance company. Note the separate user views that make up the external
schema.
The Conceptual Schema
This is another level of the design. Let us step back to the concept of external
schema. We described the external schema as the collection of data views of
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