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The Structured Query Language
INTRODUCTION
Database is an organised set of information about an entity having controlled redundancy and serves numerous applications. DBMS (database management system) is application software that is developed to make and influence the data in database. A query language can simply access a data in a database. SQL (Structured Query Language) is language used by most relational database systems. IBM developed the SQL language in mid-1979. All communication with the RDBMS and the clients or among RDBMS is by SQL. Whether the client is a disguised engine or a basic SQL engine such as a GUI, one RDBMS talking to another or report writer, SQL statements pass from the client to the server. The server responds by processing the SQL and returning the results. The benefits of this approach are that the only network traffic is the first query and the resulting response. The processing power of the client is reserved for running the application.
SQL is a data sub-language having of three built-in languages: Data definition language (DDL), Data manipulation language (DML) and Data control language (DCL). It is a fourth generation language. SQL has many more benefits and features. It should be noted that a lot of commercial DBMS may or may not execute all the details given in this unit. For instance, MS-ACCESS does not support some of these features. Even some of the constructs may not be portable, please consult the DBMS Help for any such dissimilarity.
Sub query in the UPDATE command : Example : //Double the commission for employees, who have got at least 2 increments. UPDATE EMP SET COMM = COMM * 2 WHERE
Objectives After reading this unit you should be able to: Explain storage of databases on hard disks; Discuss the execution of various file Organisation techniques;
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