Organizing and Accessing Stored Data

As user  of computers, data as we are instructed to do, such as punching in our identification code at an automated teller machine or perhaps  filling out a from with our name and address. but data cannot be dumpet helter-skelter into a computer. Some computer profesional- probably a programmer or systems analyst-had to plan how data from users will be  received, organized, and stored, and also in what manner data will be processed by the computer. First consider how data is organized for processing.

Data: Getting Organized

To be processed by the computer, raw data is organized into characters, fields, records, files, and data bases. We will start with the smallest element, the character.

• A Character is a letter, digit, or special character (such as $, ?, or *).
• A field contains a set of related characters. For example, suppose a health club is making address labels for a mailing. For each person it might have a member-number field, a name field, a street-address field,, a city field, a state field, a zip-code field, and a phone number field.
• A record is a collection of related fields. Thus, on the health club mailing list, one person's member number, name, address, city, state, zip code, and phone number constitute a record.
• A file is a collection of related record. All the member records for the health club compose a file.
• A database is a collection of interrelated file stored together with minimum redundancy. Spesific data items can be retrieved for various applications. For instance, if the health club is opening a new outlet, it can pull out the names of people with zip codes near the new club, and send them an announcement. 

Disk

A field of particular interest is the field called the key, a unique identifier for a record. It might seem, at first, that name-of a person, say, or a product-would be a good key. When a file is first computerized, exiting description fields are seldom used as keys. Although a file describing people might use a Social Security number as the key, it is more likely that a new field will be developed that can be assigned unique values, such as customer number or product number.

In addition to organizing the expected data, a plan must be made up access the data on files.

The File Plan: An Overview

Now that we have a general idea of how data is organized, we need to consider what way would be appropriate to place data on dtorage medium-tape or disk. 

Consider this chain :

1. It is a application-payroll, airline reservations, inventory control, whatever-that determines the way the data must be accessed by users.
2. Once an access method has been determined, then it follows that there are certain ways the data must be organized so that the needed access is workable.
3. The organization method, in turn, limits what storage medium may be used. we will discuss both organization and access in detail, but let us begin with an appreciation of application demands.

Consider these application examples to see how an access decision migh be made.

1. A departement store offers its customers charge accounts. When a customer makes a purchase, a sales clerk needs to be able to check the validity of the customer's account while the customer is waiting. The clerk needs immediate access to the individual customer record in the account file.
2. A major oil company supplies its charge customers with credit cards, wich it considers  sufficient proof for purchase.. The charge slips colectec by gas stations are forwarded to the oil company, which processes them in order of account number. Unlike the retail example above, the company does not need access to any one record at a specific time but merely needs access to all customer charge records when it is time to prepare bills.
3. A city power and light company employee accepts reports of burned-out streetlights from residents over the phone. Using a key made up of unique address components, the clerk immediately  finds the record for the offending streetlight and print out a one-page report that is routed to repair units within 24 hours. To product such quick service based on an individual steetlight, the employee needs to be able to access the individual streetlight record.
4. Reports of airline fligh attendant next-month schedules are computer-produced monthly and delivered to the attendant's home-base mailboxes. The schedules are put together from information based on flight records, whose entire file can be accessed monthly at the convenience of the airline and the computer-use plan.

As you can see, the question of access seems to come down to whether a particular record is needed right q way, as we will see, this immediate need for a particular record means access must be direct. It follows that the organization must also be direct, or at least indexed, and that the storage medium must be disk. Furthermore, the type of processing, a related topic, must be transaction processing. There are other possibilities, but the critical distinction is wether or not access to an individual record is needed. We now examine all these topics in detail. Although, as we have just seen, organization type is determined by the type of access required, the file must be organized before it can be accessed, so we begin there.

Share this post