DATA LANGUAGES-DAY7

Data Definition Language(DDL)

The data definition language (DDL) is the language used to define and modify the conceptual scheme of the database.

· Often used to define/modify sub schemes

· The definition of the conceptual scheme (written in the DDL) is compiled into a file called the data dictionary

· The data dictionary is consulted for scheme information whenever data is read or modified

· A separate language called the data storage and definition language is used to access the physical scheme of the database

Data Manipulation Language (DML):

The language used at the conceptual and view levels to retrieve, insert, delete and modify information stored in the database

Query

A request for information retrieval. Example (in English): List all customers who have an account balance over $500

Query language

The part of the DML used for retrieving information

The terms query language and data manipulation language are often used interchangeably.

Example: SQL is a DML, but is usually called a query language.

DML's are classified by the kind of query language they include:

· Procedural DML's require the user to specify what data is needed and to provide an algorithm for getting the data. The algorithm is written in query language.
Examples: SQL, Quell

· Nonprocedural DML's require only that the user specify what data is needed
Examples: Data log, QBE

Notes:

· Like the distinction between procedural programming languages (C, C++, Ada) and nonprocedural languages (Prolog)

· DDL's manipulate schemes, while DML's manipulate instances