What happens if you attempt to set a column to NOT NULL and the column contains NULL values?

• A. NULL values are changed to 0
• B. NULL values are changed to an empty string
• C. Snowflake returns an error
• D. Snowflake deletes the rows with NULL values

Answer: (C)

Setting a NOT NULL constraint on a column requires all existing values to be non-null. If there are NULLs present, Snowflake cannot enforce the constraint, so the DDL operation fails with an error. The system does not automatically convert NULLs to 0 or an empty string; such automatic data transformation would change the actual data, which isn’t allowed here.

To proceed, update the table to replace NULLs with a valid non-null value (for example, a default you choose), or remove those rows, and then re-run the ALTER TABLE to add the NOT NULL constraint. Once there are no NULLs, the constraint can be applied successfully. If you also want future inserts to supply a value automatically, you can define a DEFAULT for the column or enforce NOT NULL after cleaning the data.