DDL is used to specify database schemas. The result
of compilation of DDL statements is set of tables that are stored in a special
file called data dictionary or data directory. A data directory is a file that
contains data that is, data about data. This file is consulted before actual
data are read or modified in the database system. Storage structure and access
methods used by database system are specified by a set of definitions in a
special type of DDL called a data storage and definition language. The result
of compilation of these definitions is a set of instructions to specify the
implementation details of the database schemas details are usually hidden from
the users.
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