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Tuesday, August 4, 2009

Database (DBMS) Languages

Once the design of the database is completed and a DBMS is chosen to implement the database, the next step is to define the conceptual and internal schema for the data and any mappings between the two. The languages that are used to do so are :

• Data Definition Language (DDL): To specify the conceptual schema of a database by the DBA and by database designers. In many DBMSs, the DDL is also used to define internal and external schemas (views).
• In some DBMSs, separate storage definition language (SDL) are used to define internal and external schemas.
The mappings between the two schemas may be specified in either one of these languages. For a true three-schema architecture, a third language is needed, the view definition language (VDL), to specify user views and their mappings to conceptual schema.

Once the database schemas are compiled and the database is populated with data, users must have some means to manipulate the data.
• Data Manipulation Language (DML): Used to specify database retrievals and updates.
• DML commands (data sublanguage) can be embedded in a general-purpose programming language (hostlanguage), such as COBOL, C or an Assembly Language.
• Alternatively, stand-alone DML commands can be applied directly (query language).
DMLs can be high level(set-oriented, nonprocedural) or low level)record-oriented, procedural). A high-level DML can be embedded in a host programming language, or it can be used as a stand-alone language; in the latter case it is often called as a query language.

In current DBMSs, a comprehensive integrated language is used that includes constructs for conceptual schema definition, view definition, and data manipulation. A typical example of a comprehensive database language is the SQL relational database language.
Whenever DML commands are embedded in a general-purpose programming language, that language is called the host language and the DML is called the data sublanguage.

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