Integrity in SQL: Which statement about constraints is true?

Integrity in SQL: Which statement about constraints is true?

• A. Check constraints always enforce cross-table validation
• B. Primary keys may be null
• C. Foreign keys may reference any column
• D. Check constraints can enforce that a value falls within a permitted range

Answer: (D)

Check constraints enforce per-row logic, such as ensuring a numeric value falls within a permitted range. For example, a constraint like CHECK (age BETWEEN 0 AND 120) guarantees that any inserted or updated row has a valid age. This is why a range check is a valid use of a check constraint. Check constraints don’t perform cross-table validation; they evaluate only the values in the single row being modified, so they can’t enforce rules that depend on other rows or tables. Primary keys cannot be null; they are used to uniquely identify each row, so null values would violate that uniqueness. Foreign keys reference only columns that have a unique constraint (typically the primary key) in the referenced table, not arbitrary columns.

Check constraints enforce per-row logic, such as ensuring a numeric value falls within a permitted range. For example, a constraint like CHECK (age BETWEEN 0 AND 120) guarantees that any inserted or updated row has a valid age. This is why a range check is a valid use of a check constraint.

Check constraints don’t perform cross-table validation; they evaluate only the values in the single row being modified, so they can’t enforce rules that depend on other rows or tables.

Primary keys cannot be null; they are used to uniquely identify each row, so null values would violate that uniqueness.

Foreign keys reference only columns that have a unique constraint (typically the primary key) in the referenced table, not arbitrary columns.