that4loopgirl

The E – R data model employs three notations:  Entity sets,  Relationship sets, and  Attributes. 1.  Entity sets An entity is a thing or object in real world that is different from other objects. For example, each employee in a company is an entity. An entity has a set of properties, and unique value for some properties. For example, the employee may have employee_id property whose value uniquely identifies the employee. Thus, 0201 value for employee_id would uniquely identify the employee in a company. An entity set is a set of entities of the same type that may share same properties, or attributes. For example, the set of all persons who are employees at a company can be defined as the entity set employee. Entity sets do not need to be disjoint. For example, a person may be an employee entity, a customer entity, both or neither. An entity is represented by set of attributes . Attributes are properties possessed by each member of the entity set. Each entity may

  Database applications are usually divided into two or three parts. In a two – tier architecture , the client machine uses query language statement and invokes database system functionally at the server machine. For interaction between the client and the server, application program interface like JDBC and ODBC are used. In a three – tier architecture , the client machine acts as a front end and do not directly call database. Instead, it communicates through forms interface with an application server, and the application server communicates with the database to access data. Figure – 1: Two – tier and three – tier architecture

The functional components of the database system can be divided into two components: the Storage manager and the Query processor.  1.   Storage Manager A storage manager is a program which provides interface between low – level data, the application programs and queries submitted to the system. It also interacts with the file manager. The storage manager translates various DML statements into low – level file – system commands. It is also responsible for storing, retrieving, and updating the data stored in the database. The components of storage manager include:  Authorization and integrity manager : checks the authority of the users to access the data and tests for the satisfaction of integrity constraints.  Transaction manager : ensures consistency of database despite system failures, and concurrent transaction execution without conflict.  File manager : manages space allocation on disk storage and data structures used to represent the information stored on disk.  Buffer manager :