Which statement best describes the Not Null constraint in SQL?

Which statement best describes the Not Null constraint in SQL?

• A. It is a table-level constraint
• B. It allows NULL values by default
• C. It prevents NULL values in the column
• D. It defines a default value for a column

Answer: (C)

Not Null makes a column mandatory by disallowing NULLs. When a column is defined with NOT NULL, every row must have a real, non-NULL value for that column; attempting to insert or update with NULL is rejected by the database. This constraint is typically specified at the column level in the table definition, and it does not itself define a default value—that’s what a DEFAULT clause is for. You can combine NOT NULL with DEFAULT to ensure a value is supplied if one isn’t provided, and a PRIMARY KEY also implies NOT NULL (plus uniqueness). So the statement that describes the constraint as preventing NULL values in the column is the correct one.

Not Null makes a column mandatory by disallowing NULLs. When a column is defined with NOT NULL, every row must have a real, non-NULL value for that column; attempting to insert or update with NULL is rejected by the database. This constraint is typically specified at the column level in the table definition, and it does not itself define a default value—that’s what a DEFAULT clause is for. You can combine NOT NULL with DEFAULT to ensure a value is supplied if one isn’t provided, and a PRIMARY KEY also implies NOT NULL (plus uniqueness). So the statement that describes the constraint as preventing NULL values in the column is the correct one.