NOT NULL is categorized as which type of constraint?

NOT NULL is categorized as which type of constraint?

• A. Table constraint
• B. Primary key constraint
• C. Unique constraint
• D. Column constraint

Answer: (D)

The key idea is that NOT NULL is a constraint tied to a single column. When you define a column as NOT NULL, you’re saying that every row must have a value for that specific column; it’s declared as part of the column definition itself. That makes it a column constraint. Table constraints, on the other hand, apply to the table as a whole or involve multiple columns (for example, a composite primary key, a multi-column UNIQUE constraint, or a CHECK that references more than one column). NOT NULL doesn’t involve multiple columns and doesn’t describe a row-wide or multi-column rule, so it isn’t a table constraint. Some other constraints can be defined at either level (like PRIMARY KEY or UNIQUE), but NOT NULL is uniquely a column-level constraint because its scope is the individual column’s nullability.

The key idea is that NOT NULL is a constraint tied to a single column. When you define a column as NOT NULL, you’re saying that every row must have a value for that specific column; it’s declared as part of the column definition itself. That makes it a column constraint.

Table constraints, on the other hand, apply to the table as a whole or involve multiple columns (for example, a composite primary key, a multi-column UNIQUE constraint, or a CHECK that references more than one column). NOT NULL doesn’t involve multiple columns and doesn’t describe a row-wide or multi-column rule, so it isn’t a table constraint.

Some other constraints can be defined at either level (like PRIMARY KEY or UNIQUE), but NOT NULL is uniquely a column-level constraint because its scope is the individual column’s nullability.