DDL

• Data Definition Language (DDL)

  • Overview

    • Data Definition Language is the part of a database language used to define and change database structures. In SQL, CREATE TABLE, ALTER TABLE, and DROP TABLE are DDL statements. They describe the shape and constraints of data rather than individual row values.

  • Why it matters

    • DDL turns a Database/Schema into structures a database system can enforce: tables, columns, data types, keys, and constraints. Changing those definitions changes which data the system accepts and which queries applications can make.
    • A schema change may require existing data to be checked or transformed. The effects of a DDL statement on locks, transactions, and running queries depend on the database engine.

  • Place in database systems

    • Database/System/Theory considers the relationship between the logical data model, queries, transactions, and physical storage. DDL expresses part of that logical model in a language the system can execute.