7. Data Description Language

Sometimes called the 'Data Definition Laguage' or DDL. The DDL is part of a Database Management System.

The DDL provides the database designer with a way of describing the architecture of the database itself. This is called the 'schema' of the database.

The DDL allows tables to be created that describes

• the structure of each data table
• define primary and secondary keys
• the fields / attributes used within those tables
• the data types and length of each field
• the security / access details of each user
• any validation rules

The technical term for this 'data-about-data' is 'metadata'.

A typical DDL statement is shown below - this happens to be the commands to set up a table in Oracle, which is a powerful enterprise-class DBMS.

CREATE TABLE EMPLOYEE (
     EMP_ID NUMBER(8),
     LNAME VARCHAR2(30),
     FNAME   VARCHAR2(15),
     HIRE_DT DATE,
     SALARY NUMBER(8,2) )
     PCTFREE 20
     PCTUSED   50
     STORAGE (
     INITIAL 200K NEXT 200K
     PCTINCREASE 0 MAXEXTENTS 50   )
     TABLESPACE ts01
     LOGGING ;

Some database management systems provide an user-friendly graphical interface to the underlying DDL such as the one below.

When the designer needs to limit what a certain group of users can see \ do whilst logged on, he will create a 'sub-schema' just for that group.

The sub-schema determines which tables that particular group can view or change. For example a group called 'receptionists' could be included that limits receptionist staff to only see Location and Telephone number details of each employee and nothing else. Whereas the 'HR group' can see employment details and so on.