In a sequential file records are stored one after another in an ascending or descending order determined by the key field of the records. In payroll example, the records of the employee file may be organized sequentially by employee code sequence.
Sequentially organized files that are processed by computer systems are normally stored on storage media such as magnetic tape, punched paper tape, punched cards or magnetic disks. To access these records, the computer must read the file in sequence from the beginning. The first record is read and processed first, then the second record, in the file sequence, and so on.
To locate a particular record, the computer program must read in each record in sequence and compare its key field to the one that is needed. The retrieval search ends only when the desired key matches with the key field of the currently read record. On an average, about half the file has to be searched to retrieve the desired record from a sequential file.
Advantages of sequential files
- Easy to organize, maintain and understand
- There is no overhead in address generation. Locating a particular record requires only the specification of the key field
- Relatively inexpensive I/O Media and devices can be used for the storage and processing of such files
- It is the most efficient and economical file organization in case if applications in which there are large number of file records to be updated at regularly scheduled intervals.
Disadvantages of sequential files
- It proves to be very inefficient and uneconomical for applications in which the activity ratio is very low
- Since an entire sequential file may need to be read just to retrieve and update few records, accumulation of transactions into batches is required before processing them
- Transactions must be stored and placed in sequence prior to processing
- Timeliness of data in the file deteriorates while batches are being accumulated
- Data redundancy is typically high since the same data may be stored in several files sequenced on different keys
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