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Data-Definition Language

We specify a database schema by a set of definitions expressed by a special language called a data-definition language (DDL).  

For instance, the following statement in the SQL language defines the account table: 

create table account (account-number char(10), balance integer) 

Executionn of the above DDL statement creates the account table. In addition, it updates a special set of tables called the data dictionary or data directory.  

A data dictionary contains metadata—that is, data about data. The schema of a table is an example of metadata. A database system consults the data dictionary before reading or modifying actual data.  

We specify the storage structure and access methods used by the database system by a set of statements in a special type of DDL called a data storage and definition language.  

These statements define the implementation details of the database schemas, which are usually hidden from the users.  

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