How do you ensure a column cannot contain NULL values at creation time?

• A. column_name datatype NOT NULL
• B. column_name datatype NULL
• C. column_name datatype NOT NULL DEFAULT NULL
• D. column_name datatype PRIMARY KEY

Answer: (A)

Adding a NOT NULL constraint to the column is the way to ensure it can’t be NULL from creation time. This tells the database that every row must have a real value for that column, and any insert or update that would set it to NULL is rejected. Although PRIMARY KEY also disallows NULLs, it also requires the column to be unique and identifies rows, which is a stronger, different constraint than simply preventing NULLs. The other option with DEFAULT NULL would allow NULLs if no value is provided, which defeats the purpose. So the correct approach is to declare the column as datatype NOT NULL.