Databases
When computers were first used for data processing each application would have its own data file. Thus in a company selling goods by mail order the customer accounts program, the stock control program, the payroll program would each have its own file. This leads to duplication of data in different files. The accounts file would contain data about customers and stock, which would be duplicated in the customer and stock files respectively. When this happens we say that there is data redundancy in the system. Application specific files can be represented as two dimensional tables and are therefore called flat files.
Data redundancy creates the following problems:
- Storage space is not being used efficiently, but more importantly
- If the same data is in a number of locations then a change that data requires a number of different files to be updated. This might not be possible at a particular time leading to inconsistency of data where the same data item has different values in different parts of the system.
- In addition data redundancy can also lead to data inconsistency because the same item of data will need to be entered in the different files. This could lead to an account number or name being typed wrongly.
A database attempts to reduce data redundancy by integrating the data in the different files. The database in managed by a Database Management System. Application programs access the data through the database management system, which is a layer of software between the data and the application. All access to the data and all editing will be through the DBMS. In addition to providing logical record data to application rograms the DBMS will also be responsible for the following system functions:
- Maintaining data by adding new records, deleting old records and amending the data stored in a record.
- Providing data security by protecting it against unauthorised access and corruption.
- Handling error situations and recovering from them - e.g. loss of power.
- Maintaining access statistics for billing purposes.
The Database Approach aims to achieve the following:
| Avoid data redundancy | by replacing redundant data by link fields pointing to the appropriate record in another file. |
| Maintain data consistency | by reducing data redundancy |
| Provide data independence | i.e. ensure that the way that the data is stored does not depend on the needs of any one application. This is achieved by the DBMS collecting data together from the physical records to form a logical record that is then presented to a particular application program. A logical record may be composed of fields from several different physical records in different files. The DBMS provides the application with a view of the data that makes it look as though the record structure is exactly as required for that application. Each application that uses the data sees logical records that exactly match its needs. The underlying data structure is however independent of any one application's specific needs. |
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