A introduction to Database Management System Raghu Ramakrishnan

A introduction to Database Management System Raghu Ramakrishnan Wiley[U.S.] A database (sometimes spelled data base) is also called an electronic database , r eferring to any collection of data, or information, that is specially organized for rapid search and retrieval by a computer. Databases are structured to facilitate the storage, r etrieval , modification, and deletion of data in conjunction with various data-processin g operations .Databases can be stored on magnetic disk or tape, optical disk, or some other secondary storage device. A database consists of a file or a set of files. The information in these files may b e broken down into records, each of which consists of one or more fields. Fields are th e basic units of data storage , and each field typically contains information pertaining to one aspect or attribute of the entity described by the database . Using keywords and various sorting commands, users can rapidly search , rearrange, group, and select the fields in many records to retrieve or create reports on particular aggregate of data. Complex data relationships and linkages may be found in all but the simplest dat abases .The system software package that handles the difficult tasks associated with cr eating ,accessing, and maintaining database records is called a database management s ystem(DBMS).The programs in a DBMS package establish an interface between the d atabase itself and the users of the database.. (These users may be applications program mers, managers and others with information needs, and various OS programs.) A DBMS can organize, process, and present selected data elements form the dat abase. This capability enables decision makers to search, probe, and query database c ontents in order to extract answers to nonrecurring and unplanned questions that aren’t available in regular reports. These questions might initially be vague and/or poorly defined ,but people can “browse” through the database until they have the needed info rmation. In short, the DBMS will “manage” the stored data items and assemble the ne eded items from the common database in response to the queries of those who aren’t programmers. A database management system (DBMS) is composed of three major parts:(1)a s torage subsystem that stores and retrieves data in files;(2) a modeling and manipulatio n subsystem that provides the means with which to organize the data and to add , delet e, maintain, and update the data;(3)and an interface between the DBMS and its users. Several major trends are emerging that enhance the value and usefulness of database management systems; Managers: who require more up-to-data information to make effective decision C ustomers: who demand increasingly sophisticated information services and more curr ent information about the status of their orders, invoices, and accounts. Users: who fin d that they can develop custom applications with database systems in a fraction of the time it takes to use traditional programming languages. Organizations : that discover information has a strategic value; they utilize their database systems to gain an edge over their competitors. The Database Model A data model describes a way to structure and manipulate the data in a database. The structural part of the model specifies how data should be represented(such as tree , tables, and so on ).The manipulative part of the model specifies the operation with w hich to add, delete, display, maintain, print, search, select, sort and update the data. Hierarchical Model The first database management systems used a hierarchical model-that is-they arr anged records into a tree structure. Some records are root records and all others have u nique parent records. The structure of the tree is designed to reflect the order in which the data will be used that is ,the record at the root of a tree will be accessed first, then records one level below the root ,and so on. The hierarchical model was developed because hierarchical relationships are co mmonly found in business applications. As you have known, an organization char ofte n describes a hierarchical relationship: top management is at the highest level, middle management at lower levels, and operational employees at the lowest levels. Note that within a strict hierarchy, each level of management may have manyemployees or leve ls of employees beneath it, but each employee has only one manager. Hierarchical dat a are characterized by this one-to-many relationship among data. In the hierarchical approach, each relationship must be explicitly defined when th e database is created. Each record in a hierarchical database can contain only one key field and only one relationship is allowed between any two fields. This can create a pr oblem because data do not always conform to such a strict hierarchy. Relational Model A major breakthrough in database research occurred in 1970 when E. F. Codd pr oposed a fundamentally different approach to database management called relational model ,which uses a table as its data structure. The relational database is the most widely used database structure. Data is organi zed into related tables. Each table is made up of rows called and columns called fields . Each record contains fields of data about some specific item. For example, in a table containing information on employees, a record would contain fields of data such as a person’s last name ,first name ,and street address. Structured query language(SQL)is a query language for manipulating data in a re lational database .It is nonprocedural or declarative, in which the user need only speci fy an English-like description that specifies the operation and the described record or combination of records. A query optimizer translates the description into a procedure t o perform the database manipulation. Network Model The network model creates relationships among data through a linked-list structu re in which subordinate records can be linked to more than one parent record. This ap proach combines records with links, which are called pointers. The pointers are addres ses that indicate the location of a record. With the network approach, a subordinate re cord can be linked to a key record and at the same time itself be a key record linked to other sets of subordinate records. The network mode historically has had a performan ce advantage over other database models. Today , such performance characteristics ar e only important in high-volume ,high-speed transaction processing such as automatic teller machine networks or airline reservation system. Both hierarchical and network databases are application specific. If a new applic ation is developed ,maintaining the consistency of databases in different applications can be very difficult. For example, suppose a new pension application is developed .T he data are the same, but a new database must be created. Object Model The newest approach to database management uses an object model , in which re cords are represented by entities called objects that can both store data and provide me thods or procedures to perform specific tasks. The query language used for the object model is the same object-oriented progra mming language used to develop the database application .This can create problems b ecause there is no simple , uniform query language such as SQL . The object model is relatively new, and only a few examples of object-oriented database exist. It has attrac ted attention because developers who choose an object-oriented programming languag e want a database based on an object-oriented model. Distributed Database Similarly , a distributed database is one in which different parts of the database r eside on physically separated computers . One goal of distributed databases is the acce ss of information without regard to where the data might be stored. Keeping in mind t hat once the users and their data are separated , the communication and networking c oncepts come into play . Distributed databases require software that resides partially in the larger compute r. This software bridges the gap between personal and large computers and resolves th e problems of incompatible data formats. Ideally, it would make the mainframe databa ses appear to be large libraries of information, with most of the processing accomplish ed on the personal computer. A drawback to some distributed systems is that they are often based on what is c alled a mainframe-entire model , in which the larger host computer is seen as the mast er and the terminal or personal computer is seen as a slave. There are some advantage s to this approach . With databases under centralized control , many of the problems o f data integrity that we mentioned earlier are solved . But today’s personal computers, departmental computers, and distributed processing require computers and their applic ations to communicate with each other on a more equal or peer-to-peer basis. In a data base, the client/server model provides the framework for distributing databases. One way to take advantage of many connected computers running database appl ications is to distribute the application into cooperating parts that are independent of o ne anther. A client is an end user or computer program that requests resources across a network. A server is a computer running software that fulfills those requests across a network . When the resources are data in a database ,the client/server model provides t he framework for distributing database. A file serve is software that provides access to files across a network. A dedicate d file server is a single computer dedicated to being a file server. This is useful ,for ex ample ,if the files are large and require fast access .In such cases, a minicomputer or mainframe would be used as a file server. A distributed file server spreads the files ar ound on individual computers instead of placing them on one dedicated computer. Advantages of the latter server include the ability to store and retrieve files on ot her computers and the elimination of duplicate files on each computer. A major disad vantage , however, is that individual read/write requests are being moved across the n etwork and problems can arise when updating files. Suppose a user requests a record f rom a file and changes it while another user requests the same record and changes it to o. The solution to this problems called record locking, which means that the first requ est makes others requests wait until the first request is satisfied . Other users may be a ble to read the record, but they will not be able to change it . A database server is software that services requests to a database across a networ k. For example, suppose a user types in a query for data on his or her personal comput er . If the application is designed with the client/server model in mind ,the query lang uage part on the personal computer simple sends the query across the network to the d atabase server and requests to be notified when the data are found. Examples of distributed database systems can be found in the engineering world. Sun’s Network Filing System(NFS),for example, is used in computer-aided engineeri ng applications to distribute data among the hard disks in a network of Sun workstatio n. Distributing databases is an evolutionary step because it is logical that data should exi st at the location where they are being used . Departmental computers within a large c orporation ,for example, should have data reside locally , yet those data should be acc essible by authorized corporate management when they want to consolidate departme