Data Item: The smallest unit of data that has meaning to its users is called a data item. (or field in COBOL elementary item or data element)
Records: A records is a collection of inter related data items.
Data Aggregate (Segment): Collection of data items within records is called data aggregate or segment. Ex: A data aggregate date may be composed of data items months, date and year.
File: A file is collection of similar type of records.
* Characteristics of Database (Or Objective of DB Organization Or Database):
Back Ground: Before the evolution of database, data was processed by batch processing. Data was divided into separate files for separate applications. Redundancy, data-dependences, inconsistency, slow searching, slow query response and inefficient storage space utilization were some of the drawbacks. Data base systems evolved in about 1970.
The ultimate objective of the database systems is to make application development easier, cheaper, faster and more flexible. An ideal database system has the following characteristics or objectives (which are the advantage of database systems over separate file systems).
(1) Controlled Redundancy: Prior to the use of database technique, a large amount of data redundancy existed in stored data. As data was stored in separate files for separated applications many data items were stored redundancy in various files. The redundancy grew with each update of data. Redundant data are expensive as they required more storage than is necessary. Different copies of data may be in different stages of updating, because of this, the system may give conflicting information. The objective of the data base is to move much of this redundancy. It is difficult to make a completely non-redundant data base. In reality, some amount of redundancy exists. This improves the access times and is duplicated to prevent from accidental loss of data. Thus, there is a off between non-redundancy and other desirable criteria. The objective of data base is to remove harmful redundancy and to have controlled redundancy.
(2) Consistency (i.e. remove inconsistency): This is a corollary of 1 suppose some data (say, employee E2 works in depth D4) exists at two different files in a database system. Suppose an update is to be made (say, Employee E2 now works in depth D5). If the system is unaware of this duplication, update will be made only at one place. Thus after update there will be two entries which will not agree. At such times, the database is said to be inconsistent. The objective of database system is to remove inconsistencies, there by preventing conflicting and incorrect information.
(3) Minimum Cost: since data base redundancy, the storage cost is minimized. Less storage means less data entry, which minimizes the data entry cost. Logical data organization is designed to keep the application programming simple. This minimizes the cost of programming.
(4) Data Independence: Is means that the data and application program which use them are independent, so that it may be changed without changing the other. The objective of the data base systems is to provide data independence. Data independence simplifies the programs task. There are two levels of data independence. Logical data independence means that the overall logical structure of the data or application program. Thus, there is no need to change programs if files migrate from one type of storage device to another or the file structure is changed. Data independence reduces program.
(5) Share Data: The data in the database can be shared by any number of application programs or users. Some data bases support concurrent sharing, i.e. a same piece of data may be used by several users at the same time. A data base system supporting this from of sharing is something referred to as a multi-user system.
(6) Security and Privacy: Data is of vital important to organizations and may be confidential. The objective of the data base system is to keep the data secure and private. Security means protecting the data from accidental modification or destruction, theft, fire software or hardware failures, unauthorized persons, criminal, incompetents, person who would misuse it. Privacy means the right of an individual and organization to determine for themselves.
When how and to what extent information about them is to be transmitted to others. The database administrator (DBA) is responsible for security and privacy. It applies several authorization and identification checks to prevent unauthorized persons from using sensitive data.
(7) Integrity: Data integrity means that the data contained in the database is both accurate and consistent. The database administrator (DBA) should employ data validation procedures and check range. Checks while data is entered.
(8) Search Capability: The objective of the data base is to provide a speedy response to anticipate and unanticipated queries from users. Efficient fast search methods and adequate fast hardware is utilized for real-time response at terminals.
(9) Data migration: The process of adjusting the storage of data to suit their popularity rating is called data migration. In some systems it is done automatically, in other it is done by systems programmer or the system administrator. Some data are referenced very frequently referenced data should be stored on a storage media such as disk or drum so that it can be accessed quickly and the occasionally referenced data should be stored on inexpensive media such as magnetic tape. Date which is very important today may not remain that much important the other day. For example, suppose a flight is taking off two days after, the record of that flight and records of passenger traveling by that flight are very important. On the day of flight since there is high activity on these records, so these records should be stored on fast access storage media. On the other hand there will be less activity on these records after the take off. So this data is not so important although it should be stored. Thus it should be stored on a cheaper medium. Thus it should be stored on a changes, it should be moved from one storage to another depending on its activity.
(10) Tuning: adjusting a data to improve its performances is referred to as tuning the data base. The data base administrator or his group is responsible for tuning the database.
(11) Simplicity: Overall logical view of the data should be simple and neat. For showing relationships between data simplified logical pointers are used. But logical pointers increase complexity. Hence there should be minimum use of logical pointers.
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