How to Add a NOT NULL Constraint in Snowflake

In Snowflake, a NOT NULL constraint ensures that a column cannot contain NULL values. This constraint is essential for maintaining data integrity and preventing incomplete data entries. In this tutorial, we'll guide you through the process of adding a NOT NULL constraint to an existing table.

Step 1: Understand the Syntax

The syntax for adding a NOT NULL constraint in Snowflake is simple. You can either define the constraint while creating a new column or add it to an existing column. Here’s the basic syntax:

ALTER TABLE  MODIFY COLUMN  SET NOT NULL;

Step 2: Alter the Table to Add a NOT NULL Constraint

To add a NOT NULL constraint to an existing column, use the ALTER TABLE command. For example, if you have a table named employees and you want to ensure that the email column cannot contain NULL values, you would run:

ALTER TABLE employees MODIFY COLUMN email SET NOT NULL;

Step 3: Verify the Change

After applying the constraint, it's a good practice to verify the column's definition to ensure that the NOT NULL constraint has been correctly applied. You can use the following query to check the table's column properties:

DESCRIBE TABLE employees;

This will display the table's schema and indicate whether the NOT NULL constraint is present for the specified column.

Considerations

Before adding a NOT NULL constraint, make sure that there are no existing NULL values in the column. If there are, you will need to update those values before applying the constraint, or the operation will fail.

Conclusion

Adding a NOT NULL constraint to your Snowflake table is a simple yet powerful way to enforce data integrity. It ensures that critical columns, like those storing user emails or product identifiers, always have valid data.