SQL ALTER COLUMN

What does SQL ALTER COLUMN do?

ALTER COLUMN modifies the definition of an existing column within a table.

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Compatible dialects for SQL ALTER COLUMN: PostgreSQL, MySQL (as MODIFY), MariaDB, SQL Server, Oracle, SQLite (limited support via RENAME TABLE workflow), Snowflake, BigQuery (ALTER COLUMN).

SQL ALTER COLUMN Full Explanation

ALTER COLUMN is a clause of the ALTER TABLE statement that lets you change the attributes of an existing column without dropping and recreating the table. Typical actions include changing the data type, adjusting length or precision, setting or dropping NOT NULL and DEFAULT constraints, renaming the column, or changing collation. Most relational databases support ALTER COLUMN, but exact capabilities and required clauses vary. Some engines (e.g., PostgreSQL) require USING expressions when converting incompatible types, while others (e.g., MySQL) expect the full column definition to be restated. ALTER COLUMN is transactional in databases that support DDL transactions (PostgreSQL, SQL Server), meaning changes can be rolled back. Be mindful that altering large tables can lock the table and may rewrite data, potentially causing long-running operations.

SQL ALTER COLUMN Syntax

ALTER TABLE table_name
  ALTER COLUMN column_name [SET DATA] TYPE new_data_type [USING expression];

-- Optionally:
ALTER TABLE table_name
  ALTER COLUMN column_name SET DEFAULT default_value;
ALTER TABLE table_name
  ALTER COLUMN column_name DROP DEFAULT;
ALTER TABLE table_name
  ALTER COLUMN column_name SET NOT NULL;
ALTER TABLE table_name
  ALTER COLUMN column_name DROP NOT NULL;

SQL ALTER COLUMN Parameters

table_name (identifier) - Name of the table containing the column.
column_name (identifier) - The column to be modified.
new_data_type (data type) - Target data type or new size/precision.
expression (expression) - Expression to cast existing data (dialect-specific).

Example Queries Using SQL ALTER COLUMN

-- 1. Change a VARCHAR(50) column to VARCHAR(100)
ALTER TABLE customers
  ALTER COLUMN email TYPE VARCHAR(100);

-- 2. Convert text column to integer using CAST
ALTER TABLE orders
  ALTER COLUMN order_num TYPE INTEGER USING order_num::integer;

-- 3. Add a NOT NULL constraint
ALTER TABLE products
  ALTER COLUMN price SET NOT NULL;

-- 4. Drop a DEFAULT value
ALTER TABLE employees
  ALTER COLUMN start_date DROP DEFAULT;

Expected Output Using SQL ALTER COLUMN

The table definition is updated
Subsequent inserts/updates follow the new column rules
Existing data is converted if compatible or if USING expression provided
If incompatible, the database raises an error

Use Cases with SQL ALTER COLUMN

Expanding string length after business growth.
Tightening constraints by setting NOT NULL.
Migrating legacy columns to standardized data types.
Removing defaults that are no longer valid.
Correcting column precision for financial data.

Common Mistakes with SQL ALTER COLUMN

Forgetting to provide USING when converting incompatible types in PostgreSQL.
Omitting the full column definition in MySQL where MODIFY is required instead of ALTER COLUMN.
Altering large tables in production hours, causing locks and downtime.
Assuming the operation is immediately reversible when the engine does not support DDL rollback.

First Introduced In

SQL-92 standard (ALTER TABLE enhancements)

Frequently Asked Questions

Can I rename a column with ALTER COLUMN?

No. Renaming uses `ALTER TABLE table_name RENAME COLUMN old_name TO new_name;`.

Do I need to restate the entire column definition in MySQL?

Yes. In MySQL you must use `MODIFY column_name new_definition` and include type, nullability, and default.

How do I drop a NOT NULL constraint?

Use `ALTER TABLE table_name ALTER COLUMN column_name DROP NOT NULL;` (PostgreSQL) or `MODIFY column_name datatype NULL` (MySQL).

What happens if existing data violates the new constraint?

The database rejects the ALTER operation, raising an error unless you first clean or convert the data.