Data Hierarchy | Alternative Field Formats | Keys | Coding | Types of files and File organisation | Data Processing Cycle | General Systems Concepts |

IMPORTANT NOTE:These notes are prepared exclusively for the benefit of the students in the Department of Accounting & Law at the State University of New York at Albany. They are not to be used by others without the permission of J. Gangolly.

Data Hierarchy:

PHYSICAL                LOGICAL
--------                -------

Database                Database

Dataset (table)         Relation

Record                  Tuple (instance)

Field                   Attribute

Byte                    Character (digit, letter or special character)

Bit                     0 or 1
--------------------------------

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Alternative Field Formats:

• Numeric
• Text (alphanumeric)
• Currency
• Date/time
• Boolean
• Counter
• Blobs

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Keys:

Definition:

A Key (or candidate key) is a field (or a set of fields) in a record which uniquely determine(s) the values in the remaining fields in the record.

A record can have more than one key. when a key consists of more than one field, it is called a composite key or a concatenated key. Keys are used in retrieving information from files. The database designer must select one of the candidate keys as the primary key that will be used in retrieving information from the database. A secondary key is any key other than the primary key that might be used for retrieving information from the database.

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Coding:

• Sequence code
• Block code
• Group code
• Mnemonic code

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Types of files and File organisation:

Types of files

• Master files
• Transaction files
• Reference (table files)
• History files
• Backup files

File organisation

• Sequential
• Random access
  • Hashing routine
  • clashes
  • overflow area
• Indexed file

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Data Processing Cycle:

INPUT ----> PROCESSING ----> OUTPUT

Input:

• Batch
• On-Line

Processing:

• Batch
• On-line
• On-line Real-Time (OLRT)

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General Systems Concepts:

Updated on September 10, 1997 by Jagdish S. Gangolly.