Unit 2
Files and Databases

Records and Files

Field - group of characters e.g. surname

Record - e.g. all the information about one person

File - collection of records (extended to mean source code program written in high level language, word processed letter, bitmapped graphics file, ASCII text file

Variable length records

File Organisation

Transaction files - contain details of all transactions that have occurred in the last period

Master files - permanent files kept up-to-date by applying transactions that occur during the operation of the business

Reference file - contains data used by a program during processing




Hashing algorithm - used to translate record key into an address

Synonyms - two record keys generate the same address -> use overflow area and flag

File Processing

Operations carried out on files:

'Updating by copying' - transaction file must be sorted in same order as master file

'Updating by overlay'

File maintenance - updating more permanent fields on each record, adding / deleting records

Hit rate - proportion of records being accessed on any one run

File Security Methods

Data security - keeping data safe from the various hazards to which it mat be subjected

Call-back procedure - when a remote user logs on the computer automatically calls them back at a prearranged number to verify their access request before allowing them to log on

Methods of encryption:


Access rights

Biometric security methods

Periodic backups - copy files regularly and keep them in a safe place

Contingency plan

Data Processing Integrity Methods

Data integrity - correctness of data

'Direct data capture' - data entered automatically e.g. bar-code scanning

Hash total - sum of values calculated purely for validation purposes (use ASCII codes)

Control total - sum which has meaning

Validation checks - check data is likely

Check digit

Verification - entering data twice


Entity Relationship Modelling

Conceptual data model - describes how data elements in a system are grouped

Entity - thing of interest about which data is to be held

Attribute - property or characteristic of an entity

Relationship - link / association between entities

Database Concepts

Unproductive maintenance - programs still dependent of record structure and all departments affected by minor changes in another department

Problems of security - confidential information accessible by every application since held centrally

Database - consists of many different files linked so that information can be retrieved from several files simultaneously

Primary Key - unique identifier given to each individual record in a file

Secondary Key - not unique to each record but may be used to quickly locate a group of records

Foreign Key - key field from one table used in a second table (common field)

Index - list of numerical values which gives the order of the records when they are sorted on a particular field

Query by example (QBE)