File Organisation – SS2 Data Processing Lesson Note (Second Term)

File organisation is the logical relationship among the records that make up a file, especially how any one record is identified and accessed. The main methods are serial, sequential, indexed and random. Computer files are also classified as master, transaction or reference files.

File cabinets or catalogue drawers inside the laptop screen. Folders standing inside the drawer.

Behavioural objectives

By the end of the lesson, students should be able to:

• define file organisation and file structure;
• describe serial, sequential, indexed and random file organisation;
• classify computer files as master, transaction or reference files;
• state the criteria for classifying computer files.

What is file organisation?

File organisation refers to the logical relationships among the various records that make up a file, particularly the means of identifying and accessing any specific record.

What is file structure?

File structure refers to the format of the label and data blocks and of any logical record control information. It is used to determine an efficient file organisation for each base relation.

For example, if we want to retrieve student records in alphabetical order of name, sorting the file by student name is a good file organisation. However, if we want to retrieve all students whose marks are in a certain range, a file ordered by the student’s name would not be a good file organisation. Some file organisations are efficient for bulk loading data into the database but inefficient for retrieving and other activities.

The aim is to choose the best file organisation for each relation.

What are the types of file organisation?

Serial file

A serial file is one in which the records are stored in the order in which they arose. They have not been sorted into any particular order. An example of a serial file is an unsorted transaction file. A shopping list is an example of a non-computerised serial file. Serial files can be stored on tape, disc or in memory.

Sequential file

In a sequential file, the records are sorted and stored in a fixed order. The records are stored in physical, contiguous blocks, and within each block the records are in sequence. Records in these files can only be read or written sequentially.

Indexed file

An indexed file organisation contains reference numbers, such as employee numbers, that identify a record among other records. These references are called primary keys, and each is unique to a particular record. Alternate keys can also be defined to allow other ways of accessing the record. For example, instead of accessing an employee’s record by employee number, you can use an alternate key that references employees by department.

This gives users greater flexibility to search randomly through thousands of records in a file. However, it needs complex programming to implement.

Random file

A random file is a file organised through an index. It is also called a “direct file” or “direct access file”. It allows quick access to specific records or other elements within the file, without reading the file sequentially. The index points to a specific location within the file, and the file is read from that point.

How are computer files classified?

Master file

Master files are files of a fairly permanent nature, for example customer ledger, payroll and inventory. These files are updated regularly to show the current position. For example, when a customer’s order is processed, the “balance owing” figure on the customer ledger record increases.

Master records therefore contain both static data, such as a customer’s name and address, and data that changes each time a transaction occurs, such as the “balance” figure.

Transaction file

A transaction file is also known as a movement file. It is made up of transactions created from the source documents. In a sales ledger application, the file contains all the orders received at a particular time.

The file is used to update the master file. Once it has been used for this purpose it is no longer required. It therefore has a very short life, because it is replaced by a file containing the next batch of orders.

Reference file

A reference file is a file with a reasonable amount of permanency. Examples of data used for reference are price lists, tables of rates of pay, and names and addresses.

What are the criteria for classifying computer files?

Computer files are classified in three ways:

1. By nature of content: the kind of data the file contains.
2. By organisation method: the way the files are arranged, for example serial, sequential and random.
3. By storage medium: the storage device on which the file is kept, such as magnetic or optical disk and magnetic tape.

Evaluation

1. Define file organisation.
2. Explain the difference between a serial file and a sequential file.
3. What is a primary key? Give one example.
4. Why is a transaction file said to have a very short life?
5. State the three criteria for classifying computer files.

Assignment

1. Explain the difference between a master file and a transaction file. Give one example of each.
2. Describe how a random file gives quick access to a specific record.