Which type of integrity constraint ensures that a foreign key references a valid primary key value?

The correct answer is Option 3) Foreign key constraint.

Key Points

• A foreign key constraint is used to maintain referential integrity between two related tables.
• It ensures that the value in a column (or a group of columns) matches a valid value in the referenced column of another table—typically a primary key.
• This prevents "orphan" records (i.e., records that reference a non-existent row in another table).
• For example, if Orders.CustomerID is a foreign key referencing Customers.CustomerID, then every value in Orders.CustomerID must exist in the Customers table.

Additional Information

• Primary key constraint: Uniquely identifies each record in a table. It cannot have NULL or duplicate values.
• Check constraint: Ensures that the value in a column satisfies a specific condition (e.g., Age > 0).
• Unique constraint: Ensures all values in a column (or set of columns) are unique across rows, but unlike the primary key, it can contain NULLs.

Conclusion: A foreign key constraint guarantees that values in the referencing column exist in the referenced column, thereby ensuring referential integrity.