What is ALTER TABLE?

The ALTER TABLE statement is used to change the structure of an existing table.

It helps you update a table design after the table has already been created.

Basic ALTER TABLE syntax

The most common beginner example is adding a new column to an existing table.

ALTER TABLE table_name
ADD column_name data_type;

For example, this adds an email column to the Employees table.

ALTER TABLE Employees
ADD email VARCHAR(150);

Employees table structure before ALTER

Before running the ALTER TABLE statement, the table does not have an email column.

Adding a new column

This SQL Server-friendly example adds a new email column to the existing Employees table.

ALTER TABLE Employees
ADD email VARCHAR(150);

After this statement runs, the table has a new column available for storing employee email addresses.

What can ALTER TABLE do?

ALTER TABLE is useful when your database requirements change after a table already exists.

SQL Server notes

In SQL Server, common ALTER TABLE actions include adding columns, changing compatible column definitions, dropping columns, and adding constraints.

ALTER TABLE Employees
ALTER COLUMN department VARCHAR(100);

ALTER TABLE Employees
DROP COLUMN email;

Be careful when changing or removing existing columns because they may already contain data or be used by reports, applications, views, or stored procedures.

Key takeaway