Data Definition Language (DDL)

A database scheme is specified by a set of definitions which are expressed by a special language called a data definition language DDL the result of compilation of DDL statements is a set of tables which are stored in a special file called data dictionary or directory.

Whenever a data is read or modified in the database system, the data directory is consulted the DDL provides a set of definitions to specify the structure and access methods used by the database system.

An ideal DDL should perform the following functions:

1. It should identify the types of data division such as data item segment record and database file.
2. It should give a unique name to each data type record type file type database and other data sub division.
3. It should specify the proper data types.
4. It should specify how the record types are related to make structures.
5. It may define the type of encoding the program uses in the data items this should not be confused with encoding employed in physical representation.
6. It may define the length of the data items.
7. It may define the range of values that a data item can assume.
8. It may specify a means of checking for errors in the data.
9. It may specify a privacy locks for preventing unauthorized reading a modification of the data.
10. A logical data definition should not specify addressing indexing or searching techniques or specify the placement of data on the storage units because these topics are in the domain of physical not logical organization.