Objectives: Introduction Over View of System Analysis and Design
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- 7.9 Normalization
- 7.10 Summary
| 7.8 Entities and Attributes An entity is something of interest to the user about which to collector store data. It is also called a data aggregate because it represents a number of data elements. In our sales status system, the sales entity contains data elements such as the salesperson’s number, name and date of employment, and the sales period covered by the report. The item entity has data elements such as item number, item description, and the sale price of each item. Data entities are explained by the use of several terms attribute, value key and instance of an entity. For example, a salesperson (entity) is described by attributes such a number, name, sex, age and height. So attributes describe an entity. They are physically stored infields or data elements. Each attribute takes on a unique value. For example, “11306801” is a unique value of the attribute “ salesperson number An attribute, then, takes on a value fora specific occurrence (or instance) of an entity. A key is a unique identifier of the entity. In our example, the key 11306801 is a unique identifier of Jim Arnold. Sex, age and height are not identifiers, since they are not unique. They are non-key identifiers. 7.9 Normalization Data structuring is refined through a process called normalization. Data are grouped in the simplest way possible so that later changes can be made with a minimum of impact on the data structure. When too many attributes are grouped together to form entities, some attributes are found to be entities themselves. Further normalization of these entities into attributes linked by common data elements to form relationships improves the effectiveness of the DBMS. 7.10 Summary The design translates the system requirements into ways of operationalizing them. The design is a solution, a “ how to “ approach, compared to analysis, a what is orientation. The design phase focuses on the detailed implementation of the system recommended in the feasibility study. A file is organized to ensure that records are available for processing. It should be designed in the line with the activity and volatility of the information and the nature of the storage media and devices. There are four methods of organizing files • Sequential organization simply means storing and sorting in physical, contiguous blocks within files on tape or disk according to key. • Indexed sequential organization stores records sequentially but uses an index to locate record. Records are related through chaining using pointers. • Inverted list organizatiion uses an index for each key type. Records are not necessarily in a particular sequences. • Direct access organization has records placed randomly throughout the file. Records are updated directly and independently of the other records. A database 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. Database design minimizes the artificially embedded in using separate files. The primary objectives are fast response time to inquiries, more information at low cost, control of redundancy, clarity and ease of use, data and program independence, accuracy and integrity of the system, fast recovery, privacy and security of information, and availability of powerful end user languages. The heart of the database is DBMS. It manages and controls the database file and handle request from the application program. A data structure defines relationships among ententes. There are three types of relationships one-to-one, one-to-many and many-to-many. Although all DBMS have a common approach to data management, they are differ in the way they structure data. The three types of data structure are hierarchical , network and relational. Three are four views of data. The first three views are logical users view application program (called Subschema), and overall logical view (called Schema. The data structure can be refined through a normalization process that group the data in the simplest way possible so that later changes can be made with ease. Normalization is designed to simplify relationships and establish logical links between files without losing information. Download 0.94 Mb. Share with your friends:
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