How to Add Column in ClickHouse

Why use ALTER TABLE … ADD COLUMN?

Add new data points quickly without full-table rebuilds and keep queries backward-compatible.

What is the basic syntax?

Use ALTER TABLE table_name ADD COLUMN [IF NOT EXISTS] name type [AFTER col]. Include AFTER to control column order.

How do I add a column with a default?

Add DEFAULT or MATERIALIZED expressions to back-fill existing rows on the fly.

Can I add multiple columns at once?

Chain statements in a single command: ALTER TABLE … ADD COLUMN …, ADD COLUMN …. Each clause runs atomically.

Best practices for production tables?

Always include IF NOT EXISTS in migrations, test on a staging cluster, and use AFTER for predictable schemas.

Example 1 – Add customer_ref to Orders

ALTER TABLE Orders ADD COLUMN IF NOT EXISTS customer_ref String AFTER customer_id;

Example 2 – Add discounted_price with default

ALTER TABLE OrderItems ADD COLUMN IF NOT EXISTS discounted_price Decimal(10,2) DEFAULT price;

Example 3 – Add two columns in one shot

ALTER TABLE Products
ADD COLUMN IF NOT EXISTS is_active UInt8 DEFAULT 1,
ADD COLUMN IF NOT EXISTS updated_at DateTime DEFAULT now();

Common mistakes

Omitting IF NOT EXISTS: causes failures in idempotent deploys. Fix by adding the clause.
Ignoring column order: readers may rely on ordinal positions. Specify AFTER.