Database

Data Definition Language (DDL)

Data Definition Language (DDL)

DDL is the part of a DBMS interface enabling clients to manage metadata.

Metadata includes:

• a logical schema: logical metadata defining how data is understood
  1. There is a kind of entity called an employee.
  2. There are attributes called employee-number, name, and salary.
  3. Employees are identified by their employee-number
  4. etc.
• a physical schema: physical metadata defining how data is encoded
  • this is the concrete data structures on the server computer
• external schemata: comprised of subsets of the logical schema

So Metadata is actually at different levels of abstraction: