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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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