A database system provides a data definition language to specify the database schema and a data manipulation language to express database queries and updates. These two parts are separately two huge languages but complied together as two parts in a widely used SQL language.
Data Definition Language (DDL)
We specify a database schema by a set of definitions expressed by a special language called a Data Definition Language (DDL). Any DDL statement after creating the table schema updates a special set of tables called the data dictionary or data dictionary.
The data dictionary is called a metadata, that is ‘data of data’. A database system consults the data dictionary before reading or modifying the actual data. The storage structure and access methods used by the database system by a set of the statements in a special type of DDL is called a data storage and definition language. The data definition language thus satisfies the constraints oriented to the database schema.
Data Manipulation Languages (DML)
· The retrieval of information stored in the database.
· The insertion of new information into the database.
· The deletion of information from the database.
· The modification of information stored in the database.
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