Phase 4: Data Model Mapping (Logical Database Design)
The next phase of database design is to create a conceptual schema and
external schemas in the data model of the selected DBMS by mapping those
schemas produced in Phase 2a. The mapping can proceed in two stages:
System-independent mapping. In this
stage, the mapping does not consider any
specific characteristics or special cases that apply to the particular DBMS
implementation of the data model. We discussed DBMS-independent map-ping of an
ER schema to a relational schema in Section 9.1 and of EER schema constructs to
relational schemas in Section 9.2.
Tailoring the schemas to a specific DBMS. Different DBMSs implement a data
model by using specific modeling features and constraints. We may have to
adjust the schemas obtained in step 1 to conform to the specific implementation
features of a data model as used in the selected DBMS.
The result of this phase should be DDL (data definition language)
statements in the language of the chosen DBMS that specify the conceptual and
external level schemas of the database system. But if the DDL statements
include some physical design parameters, a complete DDL specification must wait
until after the physical database design phase is completed. Many automated
CASE (computer-aided soft-ware engineering) design tools (see Section 10.5) can
generate DDL for commercial systems from a conceptual schema design.
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