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

• no. of characters in a field varies between records
• records have varying no. of fields

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

• records are not in any particular sequence
• used as temporary files to store transaction data
• add to end
• to delete, re-write whole file to another disk

• records sorted into key sequence
• fast and efficient (may not have to read whole tape)
• to add, copy up to where record is to be inserted, insert record, then copy rest of file
• to delete, re-write whole file to another disk

• records are stored and retrieved according to their disk address / relative position within file
• to add, use the hashing algorithm to work out the appropriate memory location
• to delete, set flag = 0 but leave the value there, therefore space can be reused but is not actually empty
• to search for a record continue until either the record is found or empty space is encountered

Hashing algorithm - used to translate record key into an address

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

• interrogating / referencing
• updating
• maintaining
• sorting

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

• only one copy of file therefore security implications
• FOR RANDOM FILES ONLY since record is accessed by means of an address therefore can be written back to same address

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

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

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

• transposition -> characters are switched around
• substitution -> characters are replaced by other characters

• helps identify authentic users
• prevents alteration of the message
• prevents unauthorised users from reading the message

• read-only
• write-only
• no access
• only allow access from particular terminals or at certain times of the day

• finger-print recognition techniques
• voice recognition
• face recognition
• iris recognition technology

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

• identify alternative equipment and security facilities or implement a service agreement which provides replacement equipment if needed
• have provision for alternative communication links

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

• self-checking
• calculated from the digits of the code no.
• e.g. modulus-11 traps over 99% of all errors

Verification - entering data twice

• checks blocks of data e.g. every 256 bytes
• uses a no. of digits
• calculated by adding together the numeric value of all the bytes in a block

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

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

• combine into one table info from two or more related tables
• select field in 'Answer' table
• specify search criteria
• save query and results