these are the languages, which are used for creating database,and to manipulate the database. DBMS Facilities: Two main type of facilities are provided by a DBMS
(1)Data Definition Language(DDL): The DDL can be used to define the conceptual schema and also some details about how to implement this schema in the physical devices used to store the data. This definition includes all the entity sets and their associated attributes as well as the relationships among the entity sets. The definition also includes any constraints that have to maintained. These definitions, which can be described as metadata about the data in the database, are expressed in the DDL of DBMS and maintained in a compiled form usually as a set of tables. The compiled form of the definitions is known as data dictionary. The data dictionary contains information on the data stored in the database and is consulted by the DBMS before any data manipulation operation.
The database management system maintains the information on the file structure, the method used to efficiently access the relevant data. It also provides a method whereby the application programs indicate their data requirements. The application program could use a subset of the conceptual data definition language or a separate language. The database system also contains mapping functions that allow it to interpret the stored data for the application program.
The inter schema is specified in a some what similar data definition language called data storage definition language. The definition of the internal view is complied and maintained by the DBMS. The compiled internal schema specifies the implementation details of the internal database, including the access methods employed. This information is handled by the DBMS; the user need not be aware of these details.
(1.1)Data Manipulation Language: The language used to manipulate data in the database is called data manipulation language (DML). Data manipulation involves retrieval of data from the database, insertion of new data into the database, and deletion or modification of existing data. The query is a statement in the DML that requests of data from the database. The subset of DML used to pose a query is known as a query language. The DML provides commands to select and retrieve data from the database. Commands are also provided to insert, update and delete records.
The DML can be procedural; the user indicates not only what to retrieve but how to go about retrieving it.
Date definition of the external view in most current DBMSs is done outside the application program or interactive session. Data manipulation is done by procedure calls to the subroutines provided by a DBMS or via preprocessor statements. In an integrated environment, data definition and manipulation are achieved using a uniform set of constructs that form part of the user’s programming environment.
