Database Reference
In-Depth Information
Logical design precedes the physical design activity. In the logical design activ-
ity, you create a conceptual design, not in terms of physical files, blocks, and records,
but in terms of how data are perceived by the users and developers. If the target
DBMS is relational, data are perceived as two-dimensional tables. For a hierarchi-
cal DBMS, data are seen as being organized in the form of hierarchical segments.
In a network database environment, data are perceived as network nodes. The
tables, segments, or nodes do not represent how data are physically kept in data
storage—they just indicate how we perceive data as being organized. Logical design
consists of the tasks necessary to arrive at the design of the tables, segments, or
nodes, depending on the type of target DBMS.
SIGNIFICANCE OF LOGICAL DESIGN
Think back to the days of designing file-oriented data systems before the database
era. Considering the business processes, the analyst would prepare the layouts for
the master files and the transaction files. For an order entry application, the analyst
would design the order master file and order transaction file. He or she then defined
the necessary index files. This was the extent of the data design. No separation of
data design into logical design and physical design existed. In fact, such separation
did not make sense in a file-oriented data environment.
Although this was the best available design approach at that time, two problems
were encountered. The first problem related to the lack of data independence. Every
time changes in business conditions warranted revisions to data structures, the
design of the file layouts had to be changed. The design changes affected the phys-
ical storage of data. The second problem referred to lack of simplicity for users in
understanding the data. Users could not comprehend the technical details of phys-
ical file layouts. They need to perceive and understand data in simpler ways. They
can easily perceive data as tables, segments, or nodes—not expressed in terms of
data blocks, records, fields, storage addresses, and so on.
Logical Structure Versus Physical Structure
Database management systems provide means for separating the logical structure
and manipulation of data from their physical representation in computer hardware.
The relational revolution further confirmed the need for the separation of the
logical structure from the physical structure. Logical data design simply covers
the tasks necessary for creating the logical structure as distinct from the physical
structure.
Before proceeding further, try to get a clear picture of what we mean by the
logical and physical structures. Consider a small database system consisting of data
about customers, orders, and details of orders. This is a very simple database just for
the sake of illustration of the difference between the logical and physical structures.
Figure 11-1 illustrates this example and distinguishes between the logical and
physical structures.
This figure illustrates the logical and physical structures implementing the data-
base system based on relational technology. If the database system were to be based
on the hierarchical data model, the logical and physical structures would be differ-
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