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: