How to Add Columns to MySQL Tables with ALTER TABLE

There will likely be a point at which you will need a alter a table to add a new column in MySQL. You’ll also want to make sure you don’t mistakenly delete a bunch of data. This post walks you through how to do just that.

What is ALTER TABLE in MySQL?

Use the ALTER TABLE statement to change an existing table’s structure, including adding new columns. This command allows you to define the new column’s name, data type, and additional attributes such as NOT NULL and DEFAULT values.

Syntax for adding a column

Here’s how to add a new column to a table:

ALTER TABLE table_name
ADD COLUMN column_name data_type [constraint];

Specify the table_name where you want to add the column, name your new column with column_name, and define its data_type. Optionally, include [constraint] to apply constraints like NOT NULL, UNIQUE, or DEFAULT.

Example: Adding a single column

To add a VARCHAR column named email with a maximum length of 255 characters to a users table:

ALTER TABLE users
ADD COLUMN email VARCHAR(255);

Example: Adding multiple columns

Add multiple columns in one go by separating them with commas:

ALTER TABLE users
ADD COLUMN first_name VARCHAR(100),
ADD COLUMN last_name VARCHAR(100);

Setting default values and constraints

Specify default values or constraints directly in the column definition. For example, to add a created_at column that defaults to the current timestamp:

ALTER TABLE users
ADD COLUMN created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP;

Considerations

• Performance impact: Adding columns to large tables can temporarily affect performance and availability. Always perform these types of operations during low-traffic periods or in a staging environment first.
• Data integrity: Introducing columns with NOT NULL constraints requires either setting a default value or ensuring existing rows meet the new constraint.

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