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A file is a collection of records or documents dealing with an organization, person, area or subject. A file, in the computer world, is a self contained piece of information available to the operating system or any number of individual programs. In a computer, a file system is the collection of files, where the files are placed logically for storage and retrieval. File-Based System Advantages of File Based system • File-based systems was the first method to store data in computers. The data was stored and retrieved sequentially from the disk. • File based systems are an early attempt to computerise the manual filing system (organizing the files with the help of papers). For example, a manual file can be set up to hold all the details relating to a particular matter as a project, product, task, client or employee. In an organisation there could be such files which may be labeled and stored. • The manual filing system works good when the number of items to be stored is small. It even works quite well when the number of items stored is quite large and they are only needed to be stored and retrieved. However, a manual file system crashes when the referencing of files are not proper and processing of information in the files are carried out. Drawbacks of File Based system There were certain drawbacks in using the file-based approach which will be discussed in detail using the below example. Example: In a university, a large number of students are enrolled and they have the option of choosing various courses. The personal details of students, fees paid by them, the number and details of the courses available, the number and the details of faculty members in various departments are the separate files in the university system. Consider the effort to answer the following queries. Figure 1.0 : File Based System • Annual fees paid by the students of Computer science department. • Number of students requiring transportation facility from a particular area. • This year's revenue from students as compared to last year.
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We know that a DBMS is a collection of programs that enables users to create and maintain a database. A major purpose of a DBMS is to provide users with an abstract view of the database. This means that the system does not provide all the details of data, rather it hides the details of how data is stored and maintained. However, in order for the system to be usable, data must be retrieved efficiently. This concern leads to the design of complex data structures for the representation of data in the database. Since many database systems end users are not computer trained, the complexities are hidden from them through several levels of abstraction in order to simplify their interaction with the system. Before getting in to the details of database systems architecture, let us discuss two important terms associated with it. View: Normally, a table contains many columns and rows. Sometimes all that data interests the user, and sometimes it does not. There may be a case when only some columns or some rows of a table interest the user. To eliminate data that is not relevant to the current needs, a view is created. A view is a subset of database that an application can process. It may contain parts of one or more tables. Views are sometimes called virtual tables. To the application or the user, views behave in similar fashion as tables; however, they do not have an independent existence. Views allow the user to look at data, but they are not parts of the database. Schema: In any data model, it is important to distinguish between the description of the database and the database itself. The description of a database is called the database schema. The database schema refers to the overall structure of the database tables that store information such as user profile data, metadata or structured information. Hence, the overall logical design of the database is called as the database schema. A schema is a collection of named objects. Schemas provide a logical classification of objects in the database. A schema can contain tables, views, triggers, functions, packages, and other objects. Note that once the schema of the database is created, usually is not changed. If in case it needs to be modified, only the owner of the schema, that is, the DBA, who has access to manipulate the structure of any object in the schema, can modify it. The distinction between the logical and physical representation of data were recognized in 1978 when ANSI/SPARC (American National Standard Institute/ Standards Planning and Requirements Committee) proposed a generalized framework for database systems. According to the ANSI/SPARC DBMS report, a DBMS can be envisioned as a three-layered system: internal, conceptual, and external. 1. Internal (physical) level: this is the lowest level of abstraction and it describes how the data is physically stored and organized on the storage medium, as well as access to the data, such as through data storage in tables and the use of indexes to expedite data access. At this level, complex low-level data structures are described in detail, which deal with actual storage. The internal model separates the physical requirement of the hardware and the operating system from the data model. 2. Conceptual level: Also known as logical or community-user level, conceptual level describes what type of data is actually stored in the data base, and the relationship among the data. At this level, entire database is described in terms of small number of relatively simple structures, such as tables and constraints. Although implementation of the simple structure at the conceptual level may involve complex physical level structures, the user of the conceptual level of abstraction is the DBA, who must decide what information is to be kept in the data base. 3. External level: The external level (or application interface) is the view that the individual user of the database has. This view is often restricted view of the database and the same database may provide a number of different views for different classes of users. This level deals with the methods through which users may access the schema, such as using an input form. In general, the end users and even the application programmers are only interested in the subset of the database. To simplify their interaction with the system, the view level of abstraction is defined. Concisely, this level is concerned with the way in which the data is viewed by individual users.
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