Data Definition Language (DDL) - DBMS

Data Definition Language (DDL)

Data definition (also called description) language (DDL) is a special language used to specify a database conceptual schema using set of definitions. It supports the definition or declaration of database objects (or data element). DDL allows the DBA or user to describe and name the entities, attributes and relationships required for the application, together with any associated integrity and security constraints. Theoretically, different DDLs are defined for each schema in the three-level schema-architecture (for example, for conceptual, internal and external schemas). However, in practice, there is one comprehensive DDL that allows specification of at least the conceptual and external schemas.

Various techniques are available for writing data definition language. One widely used technique is writing DDL into a text file (similar to a source program written using programming languages). Other methods use DDL compiler or interpreter to process the DDL file or statements in order to identify description of the schema constructs and to store the schema description in the DBMS catalog (or tables), which can be understood by DBMS. The result of the compilation of DDL statements is a set of tables stored in specific file collectively called the system log (explained in Section 1.2.6) or data dictionary.

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