SAD-Database Design

Before the database concept became operational, users had programs that handled their own data independent of other users. It was a conventional file environment with no data integration or sharing of common data across applications. In a data base environment, common data are available and used by several users. Instead of each program managing its own data, data across applications are shared by authorized  users with the data base software managing the data as an entity. A program now requests data through the data base management system(DBMS), which determines data sharing.

Objectives of data base:

The general theme behind a data base is to handle information as an integrated whole. There is none of the artificiality that is normally embedded in separate files or applications. A data base is a collection of interrelated data stored with minimum redundancy to serve many users quickly and efficiently. The general objective is to make information access easy, quick, inexpensive and flexible for the user. In data base design several specific objectives are considered:

1.Controlled redundancy: Redundant data occupies space and therefore is wasteful. If versions of the same data are in different phases of updating, the system often gives conflicting information. A unique aspect of database design is storing data only once -which controls redundancy and improves system performance

2.Ease of learning and use: A major feature of a user-friendly data base package is how easy it is to learn and use. Related to this point is that a database can be modified without interfering with established ways of using the data.

3.Data independence: An important data base objective is changing hardware and storage procedure or adding new data without having to rewrite application programs. The database should be tunable to improve performance without  rewriting programs.

4.More information at low cost: Using, storing, and modifying data at low cost are important. Although hardware prices are falling, software and programming costs are on the rise. This means that programming and software enhancements should be kept simple and easy to update.

5.Accuracy and integrity: The accuracy of a database ensures that data quality and content remain constant. Integrity controls detect data inaccuracies where they occur.

6. Recovery from failure: With multiple users accessing a database, the system must recover quickly after it is down with no loss of transactions. This objective also helps maintain data accuracy and integrity.

7.Privacy and security: For data to remain private, security measures must be taken to prevent unauthorized access. Data base security means that data are protected from various forms of destruction; users must be positively identified and their actions monitored.

8.performance: This objective emphasizes response time to inquiries suitable to the use of the data. How satisfactory the response time is depends on the nature of the user- database dialogue.

The terms that are normally used in DBMS are

1.      User’s view is a profile that the user expects to see on the report.
2.      Processing refers to the changes made.
      3.    Data model is a framework of the user’s view.
      4.    Data file is the area where the file is stored.

In a data base environment, the DBMS is the software that provides the interface between the data file on disk and the program that requests processing. DBMS stores and manages data. The procedure is as follows:
1. The user requests a sales report through the application program. The application program uses a data manipulation language (DML) to tell the DBMS what is required.

2. The DBMS  refers to the data model ,which  describes the view in a language called the data definition language (DDL). The DBMS uses DDL to determine how data must be structured to produce the user’s view.

3. The DBMS requests the input/output control system(IOCS) to retrieve the information from physical storage as specified by the application program. The output is the sales report

To summarize,
1. DML manipulates data: it specifies what is required.
2. DDL describes how data are structured.
3. DBMS manages data according to DML requests and DDL descriptions..

Functions performed by the DBMS:

1. Storing ,retrieveing , and updating data.
2.Creating program and data independence. Either one can be altered independently of the other.
3. Enforcing  procedures for data integrity.
4. Reducing data redundancy. Data are stored and maintained only once.
5. Proving security facilities for defining users and enforcing authorizations. Access is limited to authorized users by passwords or similar schemes.
6. Reducing physical storage requirements by separating the logical and physical aspects of the database.
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