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Written by Administrator
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Thursday, 09 July 2009 08:54 |
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Page 6 of 17 Relational Database ComponentsDatabases employ various programming languages: - Data definition language (DDL) Defines the structure and schema of the database. The structure could mean the table size, key placement, views, and data element relationship. The schema describes the type of data that will be held and manipulated, and its properties. It defines the structure of the database, access operations, and integrity procedures.
- Data manipulation language (DML) Contains all the commands that enable a user to view, manipulate, and use the database (view, add, modify, sort, and delete commands).
- Query language (QL) Enables users to make requests of the database.
- Data control language (DCL) It defines the internal organization of the database.
- Report generator Produces printouts of data in a user-defined manner.
Structured Query Language (SQL)
It is a Query Language, which offers three classes of operators: select, project, and join. - The select operator serves to shrink the table vertically by eliminating unwanted rows (tuples).
- The project operator serves to shrink the table horizontally by removing unwanted columns (attributes).
- The join operator allows the dynamic linking of two tables that share a common column value. The join operation is achieved by stating the selection criteria for two tables and equating them with their common columns.
Most commercial implementations of SQL do not support a project operation, instead projections are achieved by specifying the columns desired in the output. This is why the Project operator is not well known as it is fading away from most databases.
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Last Updated on Friday, 28 August 2009 05:04 |
