programs or systems using different databases and files.
These databases and files are systems that are either
online or offline; at any rate, they must be online at
execution time. Under a conventional system, when the
same data was needed in SYSTEM A and in SYSTEM
B, it was usually duplicated. This redundancy of data is
not acceptable to the AIS community. The following
problems are just a few of those that exist when
redundancy of data is required:
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l
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Excess storage is required.
An excessive number of personnel is required to
handle and manipulate the data.
A greater chance of error is possible when
updating all the common data in different
databases and files.
Excessive funds are expended for report
production for management.
Excessive CPU time is expended when
collecting data for reporting.
Data integrity is harder to maintain because of
the greater chance of error.
To overcome these problems, many Navy AIS
facility are using a DBMS. This has produced a better
record in operations and productivity than its
predecessor, the file management system. DBMS
software evolved from many different software
improvements, from many different manufacturers.
None of the many DBMSs function exactly alike.
Regardless of the manufacturer’s software installed at a
particular AIS facility, a basic DBMS can be
conceptually depicted as shown in figure 3-14. Take a
few minutes to study the figure and refer to it as you
study this section. You need to understand the concepts
before we pictorially depict a DBMS execution event.
The concepts include schemas,
definition language (DDL), and
language (DML).
Schema
subschemas, data
data manipulation
A schema is a complete description of a database,
and consists of data definition language (DDL) entries.
It includes the names and descriptions of all of the areas,
set types, record types, and associated data items and
data aggregates as they exist in the database and are
known to the DBMS. In other words, it is the overall
logical database description or framework into which
values of data items can be fitted. A schema can be
viewed like the bins in a storage house holding supplies.
The schema will not change, but the data values will.
Figure 3-14.—A database management system (DBMS).
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