Database Reference
In-Depth Information
USER GROUPS
EXTERNAL SCHEMA
AgentID
AgentName
AgentAddress
AgentPhone
Agent
Management
PolicyNo
PolicyPeriod
PolicyStatus
DeductibleAmount
PolicyTerms
Premium
New Policy
Issuance
PolicyHolderName
AccidentDate
AccidentDesc
DamageDesc
DamageAmount
SettlementAmount
Claims
Processing
PolicyHolderName
Address
Phone
PolicyNo
Customer
Promotion
Figure 4-13
Auto insurance database: external schema.
individual user groups looking at the database from outside. Now let us ask the
questions. What if there is a user group interested in every data element in the data-
base? What will be the data view of such a user group? Of course, the view will com-
prise the entire database. The view comprising the whole database represents the
concept of the complete database. It is known as the conceptual schema.
Now compare the two notions of external schema and conceptual schema. The
external schema comprises individual views of the database by separate groups of
users; the conceptual schema encompasses the whole database.
You must have already guessed it: the conceptual schema may be derived from
the external schema by combining the various data views of individual user groups.
It turns out that aggregation of the external schema is a good method for design-
ing the conceptual schema.
Figure 4-14 shows the conceptual schema for the insurance database. Observe
closely how the conceptual schema evolves out of the external schema. See clearly
the similarities and differences between the two levels of design.
The Internal Schema
If the external schema and the conceptual schema represent views of the database
from outside, the internal schema may be thought of as looking at the database from
inside. From outside, the database is perceived as a collection of data structures and
their relationships. How does the database look on the inside? If you look inside
the database as it resides on physical storage, how does it appear? For this reason,
the internal schema also goes by the name of physical schema.
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