How to ADD COLUMN in Snowflake

How do you use ALTER TABLE ADD COLUMN in Snowflake?

ALTER TABLE … ADD COLUMN adds a new column to an existing Snowflake table without recreating it.

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Description

Why would I add a column in Snowflake?

Expanding a table lets you store new attributes—like a customer’s phone number—without rebuilding or re-loading data. The ALTER TABLE … ADD COLUMN command performs this change instantly and is fully transactional.

What is the basic syntax?

Use ALTER TABLE followed by the table name and one or more ADD COLUMN clauses. Each clause defines the column name, data type, default, nullability, masking policy, and an optional comment.

How do I add a single column?

ALTER TABLE Customers ADD COLUMN phone VARCHAR(20);

This creates a nullable phone column at the end of the Customers table.

How do I add multiple columns at once?

ALTER TABLE Orders ADD COLUMN shipped_at TIMESTAMP, shipping_method STRING DEFAULT 'standard';

Supply commas between column definitions; the table is locked for the minimum time needed.

Can I prevent errors if the column already exists?

ALTER TABLE Products ADD COLUMN IF NOT EXISTS weight NUMBER(8,2);

IF NOT EXISTS makes the command idempotent—useful in CI/CD pipelines.

How do I add a NOT NULL column to a populated table?

Add the column with a DEFAULT value, then alter it to NOT NULL once every row is populated.

ALTER TABLE OrderItems ADD COLUMN status STRING DEFAULT 'pending'; ALTER TABLE OrderItems ALTER COLUMN status SET NOT NULL;

What about column position?

Snowflake always appends new columns. Physical order is cosmetic; use SELECT lists to reorder output.

Best practices to follow

• Always specify NULL or NOT NULL explicitly.
• Add DEFAULTs when backfilling isn’t practical.
• Use IF NOT EXISTS in deployment scripts.
• Version-control DDL alongside application code.

Common mistakes and fixes

Missing default on NOT NULL. Snowflake errors if rows violate NOT NULL; add with DEFAULT first.
Wrong database or schema context. Prefix table names (e.g., analytics.public.Customers) or run USE commands before altering.

Need to rollback?

Immediately run ALTER TABLE … DROP COLUMN or use Time Travel to query the pre-change version.

Next steps: automate safely

Bundle ALTER statements in a transaction or deploy via pipelines that include validation queries.

Why How to ADD COLUMN in Snowflake is important

How to ADD COLUMN in Snowflake Example Usage


-- Add a NOT NULL column with default to existing Orders table
ALTER TABLE Orders
ADD COLUMN fulfillment_status STRING DEFAULT 'processing' NOT NULL;

How to ADD COLUMN in Snowflake Syntax


ALTER TABLE <database>.<schema>.<table_name>
    ADD [ COLUMN ] [ IF NOT EXISTS ]
        <column_name> <data_type>
        [ DEFAULT <expr> ]
        [ NULL | NOT NULL ]
        [ COMMENT '<text>' ]
        [ MASKING POLICY <policy_name> ]
        [ TAG <tag_name> = '<value>' ]
        [, ...];

-- Ecommerce example
ALTER TABLE analytics.public.Customers
ADD COLUMN phone VARCHAR(20) NULL COMMENT 'Customer phone number';

Common Mistakes

Adding a NOT NULL column without a DEFAULT on a non-empty table—Snowflake returns an error because existing rows violate NOT NULL. Fix by adding a DEFAULT first, then switching to NOT NULL.
Omitting IF NOT EXISTS in automation scripts—rerunning the same deployment fails if the column already exists. Add IF NOT EXISTS for idempotent DDL.