Page 97 - DCAP516_COMPUTER_SECURITY

Unit 8: Introduction to Databases

All this is called as data hierarchy because databases are composed of files, files are composed of Notes
records, records are composed of filed, fields composed of data bytes, and finally data bytes are
a group of bits. This is very well a hierarchical structure.

8.3 Data Access Methods

There are basically three types of file organization, which allow sequential access to the data in
a file and which allow direct access to the data in a file.

1. Sequential: With a sequential access file, records must be read in the same order as they
were written to the storage media. The computer begins searching for a record in the file
and then sequentially examining the next record and so on until the required record is
located. Certain storage media magnetic tape will allow only sequential access to data.
In order for a record to be found on a reel of magnetic tape, the tape must be read
sequentially, beginning with the first record.
2. Direct Access File: Direct-access file on the other hand, allows immediate direct access to
individual records in a file. There are several techniques used to accomplish direct-access
file organization. Magnetic disks, such as floppy disks, are by far the most commonly used
devices for storing direct-access files. Direct-access file organization must be used whenever
the application requires immediate access to individual records. It is widely used today
whenever the computer configuration includes CRT (Cathode Ray Tube) terminals that
display management information on demand.
3. Indexed Sequential Access File: The most common technique for accessing disk records
directly is with the use of an index. During the creation of records, the computer uses file-
handling programmed routines to establish an index on the disk itself. The index indicates
where each record is located. The concept is similar to that of the index found at the end of
book, which specifies the page where each item of information can be located.

Notes The disk index indicates the locations (i.e. track and sector) of each record and
where a particular record can be found. A key data field in each record must be indicated
by the programmer, which is used by the computer as the basis for establishing, address
information in the index.
For example, if a payroll file is stored on a disk. To access any particular record, the user
need only supply a particular key data field, such as EMPLOYEE CODE or whatever has
been defined at the time of programming. The computer then looks up the corresponding
disk location for that particular record in the index and seeks that record directly.

8.4 Data Processing

Let us examine the steps involved in data processing. You know that data as such is the term
given to all the facts that record an event, an activity or a situation. As isolated facts and figures
may not be meaningful in themselves, they can be processed in various specific ways in order to
give them a useful meaning. Data Processing embraces the techniques of sorting, relating,
interpreting and computing items of data in order to provide meaningful and useful information.
It will be evident that to arrive at certain figures, the data must be processed according to
predefined procedures and rules arranged in a specific order. It would for instance, be pointless
to add the number of lengths of timber to the number of hours worked; the answer would be
meaningless. The term, Data Processing System, relates to an assembly of those steps in logical
order. In simple terms, the steps by which data can be converted into useful information, which

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