ALTER TABLE

DDL

Modifies an existing table structure — add, drop, or modify columns and constraints.

Syntax

ALTER TABLE name ADD|DROP|MODIFY column definition

Example

ALTER TABLE users
ADD COLUMN phone VARCHAR(20);

ALTER TABLE orders
DROP COLUMN legacy_field;

About SQL ALTER TABLE

The ALTER TABLE keyword belongs to the DDL category of SQL statements. Modifies an existing table structure — add, drop, or modify columns and constraints. Understanding this command is essential for any developer working with relational databases like PostgreSQL, MySQL, SQLite, or SQL Server.

SQL (Structured Query Language) is the standard language for managing and querying relational databases. The ALTER TABLE statement is supported across all major database systems, though specific syntax may vary slightly between PostgreSQL, MySQL, MariaDB, Oracle, SQL Server, and SQLite. Always consult your database's documentation for vendor-specific features and limitations.

Best practices for using ALTER TABLE: always test queries on a development database before running them in production, use parameterized queries to prevent SQL injection, and leverage EXPLAIN to understand query performance. For complex queries, consider using CTEs (Common Table Expressions) to improve readability and maintainability.

Related SQL Keywords

CREATE TABLEDROP TABLE