Database Reference
In-Depth Information
LOGICAL
STRUCTURE
PHYSICAL
STRUCTURE
CUSTOMER
ORDER
CUSTOMER
File/blocks/records
ORDER
File/blocks/records
ORDER DETAIL
File/blocks/records
ORDER
DETAIL
Figure 11-1
Logical and physical structures.
ent. Note that the logical structure is based on the conventional data model adopted
for the implementation. The physical structure is determined by the specific DBMS
used for the particular conventional data model.
Logical Design Phase in DDLC
Let us retrace the steps discussed so far in the development of a database system.
Quickly review the DDLC steps from the beginning and note where logical design
phase fits in the DDLC. After initial planning and feasibility study, you launch
requirements analysis and definition. You have to determine exactly what informa-
tion is required to be contained in the final database system. You have gone through
the process of creating a generic data model to truly represent the real-world infor-
mation requirements. You already know the necessity for creating a generic data
model first. You know that only a generic data model unrestricted by any constraints
can truly represent all the details of real-world information.
After the creation of a generic data model, what comes next? You know that a
generic semantic data model created through a data modeling technique such as the
E-R modeling technique has no flavor of the relational, hierarchical, or network
approaches. If you are developing a relational database, your semantic data model
must be transformed into a relational data model. What is the relational data model
at this stage of the development? It does not represent how data are to be stored
in the final database system. Then what does it represent? It indicates the logical
structure, at a conceptual level, of the data perceived as two-dimensional tables.
Figure 11-2 shows the place of logical design phase in DDLC.
What is the logical design phase? It is the set of tasks in DDLC intended for the
design of the logical structure. Observe the presentation in Figure 11-2. Note the
illustration of logical design phase for the small orders database above.
Why This Phase Is Necessary
Imagine a situation in which you plan to bypass the logical design phase and go
directly to the physical design phase from requirements definition. You will be
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