2.2.1The Data Definition Language (DDL)
DDL A language that 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.
The database schema is specified by a set of definitions expressed by means of a special
language called a Data Definition Language. The DDL is used to define a schema or to
modify an existing one. It cannot be used to manipulate data.
The result of the compilation of the DDL statements is a set of tables stored in special
files collectively called the system catalog. The system catalog integrates the metadata,
that is data that describes objects in the database and makes it easier for those objects
to be accessed or manipulated. The metadata contains definitions of records, data items,
and other objects that are of interest to users or are required by the DBMS. The DBMS
normally consults the system catalog before the actual data is accessed in the database. The
termsdata dictionary and data directory are also used to describe the system catalog,
although the term ‘data dictionary’ usually refers to a more general software system than
a catalog for a DBMS. We discuss the system catalog further in Section 2.4.
At a theoretical level, we could identify different DDLs for each schema in the threelevel
architecture, namely a DDL for the external schemas, a DDL for the conceptual
schema, and a DDL for the internal schema. However, in practice, there is one comprehensive
DDL that allows specification of at least the external and conceptual
schemas.