Database Management System course

Summary • A database-managemen t system (DBMS) consists of a collection of interrelated data and a collection of programs to access that data. The data describe one particular enterprise. • The primary goal of a DBMS is to provide an environment that is both convenient and efficient for people to use in retrieving and storing information. • Database systems are ubiquitous today, and most people interact, either directly or indirectly, with databases many times every day. • Database systems are designed to store large bodies of information. The management of data involves both the definition of structures for the storage of information and the provision of mechanisms for the manipulation of information. In addition, the database system must provide for the safety of the information stored, in the face of system crashes or attempts at unauthorized access. If data are to be shared among several users, the system must avoid possible anomalous results. • A major purpose of a databas...

History of Database Systems Data processing drives the growth of computers, as it has from the earliest days of commercial computers. In fact, automation of data processing tasks predates computers. Punched cards, invented by Hollerith, were used at the very beginning of the twentieth century to record U.S. census data, and mechanical systems were used to process the cards and tabulate results. Punched cards were later widely used as a means of entering data into computers. Techniques for data storage and processing have evolved over the years: • 1950 s and early 1960s : Magnetic tapes were developed for data storage. Data processing tasks such as payroll were automated, with data stored on tapes. Processing of data consisted of reading data from one or more tapes and writing data to a new tape. Data could also be input from punched card decks, and output to printers. For example, salary raises were processed by entering the raises on punched cards and reading the pun...

Application Architectures Most users of a database system today are not present at the site of the database system, but connect to it through a network. We can therefore differentiate between clien t machines, on which remote database users work, and server machines, on which the database system runs. Database applications are usually partitioned into two or three parts, as in Figure 1.5. In a two-tie r architecture , the application is partitioned into a component  that resides at the client machine, which invokes database system functionality at the server machine through query language statements. Application program interface standards like ODBC and JDBC are used for interaction between the client and the server. In contrast, in a three-tie r architecture , the client machine acts as merely a front end and does not contain any direct database calls. Instead, the client end communicates with an applicatio n server , usually through a forms interface. The application server...

  Database System Structure A database system is partitioned into modules that deal with each of the responsibilites of the overall system. The functional components of a database system can be broadly divided into the storage manager and the query processor components. The storage manager is important because databases typically require a large amount of storage space. Corporate databases range in size from hundreds of gigabytes to, for the largest databases, terabytes of data. A gigabyte is 1000 megabytes   (1 billion bytes), and a terabyte is 1 million megabytes (1 trillion bytes). Since the main memory of computers cannot store this much information, the information is stored on disks. Data are moved between disk storage and main memory as needed. Since the movement of data to and from disk is slow relative to the speed of the cen- tral processing unit, it is imperative that the database system structure the data so as to minimize the need to move data between disk an...

Transaction Management Often, several operations on the database form a single logical unit of work. An ex- ample is a funds transfer, as in Section 1.2, in which one account (say A ) is debited and another account (say B ) is credited. Clearly, it is essential that either both the credit and debit occur, or that neither occur. That is, the funds transfer must happen in its entirety or not at all. This all-or-none requirement is called atomicity . In addition, it is essential that the execution of the funds transfer preserve the consistency of the database. That is, the value of the sum A + B must be preserved. This correctness requirement is called consistency . Finally, after the successful execution of a funds transfer, the new values of accounts A and B must persist, despite the possibility of system failure. This persistence requirement is called durability . A transactio n is a collection of operations that performs a single logical function in a database application. Each tr...

Database Users and Administrators A primary goal of a database system is to retrieve information from and store new information in the database. People who work with a database can be categorized as database users or database administrators. Database Users and User Interfaces There are four different types of database-system users, differentiated by the way they expect to interact with the system. Different types of user interfaces have been designed for the different types of users. • Naiv e users are unsophisticated users who interact with the system by invoking one of the application programs that have been written previously. For example, a bank teller who needs to transfer $50 from account A to account B invokes a program called transfer . This program asks the teller for the amount of money to be transferred, the account from which the money is to be tran ferred, and the account to which the money is to be transferred. As another example, consider a user who wishes to fi...

Database Languages A database system provides a dat a definition language to specify the database schema and a dat a manipulation language to express database queries and updates. In practice, the data definition and data manipulation languages are not two separate languages; instead they simply form parts of a single database language, such as the widely used SQL language. Data-Definition Language We specify a database schema by a set of definitions expressed by a special language called a data-definition language ( DDL ). For instance, the following statement in the SQL language defines the account table: Execution of the above DDL statement creates the account table. In addition, it up- dates a special set of tables called the data dictionary or data directory . A data dictionary contains metadat a — that is, data about data. The schema of a table is an example of metadata. A database system consults the data dictionary before reading or modifying actual data. We specify t...