DDL

Data Definition Language (DDL) is a subset of SQL used to define and manage the structure of database objects, such as schemas, tables, views, indexes, and constraints. DDL commands affect the schema rather than the data and are distinct from Data Manipulation Language (DML), which handles data queries and updates, and from Data Control Language (DCL) and Transaction Control Language (TCL).

The core DDL statements include CREATE, ALTER, DROP, TRUNCATE, and RENAME. CREATE defines new objects, such as a table or index. ALTER modifies existing objects, for example by adding or changing a column or constraint. DROP removes objects from the database. TRUNCATE quickly removes all rows from a table while preserving its structure, and RENAME changes an object's identifier. Some systems also support metadata or comment statements to annotate database objects, though the exact syntax can vary by database system.

DDL statements are executed by the database engine and modify the database schema, updating system catalogs or data dictionaries. They often require locks and can affect dependent objects, permissions, constraints, and triggers. Because they change the structure of the database, their behavior and impact are typically more disruptive than standard data updates.

Transactional behavior for DDL varies by database. In many systems, DDL is treated as an auto-commit operation and cannot be rolled back within a transaction, though some modern relational databases provide transactional DDL semantics within a single transaction boundary.

Examples include: CREATE TABLE employees (id INT PRIMARY KEY, name VARCHAR(100), hire_date DATE); ALTER TABLE employees ADD COLUMN department VARCHAR(50); DROP TABLE employees; TRUNCATE TABLE employees. Syntax and capabilities differ among database products, so consultation of vendor documentation is recommended.