Schema

SQL ALTER TABLE

Change a table definition; supported operations vary by dialect.

Availability: Common SQL with references for SQL standard, MySQL/MariaDB, PostgreSQL, SQL Server, SQLite and Oracle Database; syntax and feature support vary by engine and version.中文

Syntax

ALTER TABLE table_name ADD COLUMN column_name data_type;

Examples

Example (standard-style syntax). Verify supported keywords in your database.

ALTER TABLE users ADD COLUMN timezone VARCHAR(64);

SQLite-oriented variant; dialect behaviour may differ.

ALTER TABLE users ADD COLUMN timezone TEXT;

Parameters, options and clauses

Option / clause	Description
`Object name`	Name a table, view, index, schema or database.
`Column definition`	Specify column name, data type and default value.
`Constraints`	Use NOT NULL, UNIQUE, CHECK, PRIMARY KEY or FOREIGN KEY as needed.
`IF EXISTS / IF NOT EXISTS`	Make create/drop operations conditional where supported.
`CASCADE / RESTRICT`	Control dependent-object handling where supported; verify before destructive changes.

Dialect references

Safety note

Review a command and its target before running it. Commands that alter files, services, accounts, disks, permissions or databases may be difficult to reverse. Test in a safe environment and keep backups.

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