RDBMS Constraint: Key Attributes Cannot Be NULL

In a Relational Database Management System (RDBMS), tables use keys (specifically primary keys) to uniquely identify each record (row). A fundamental rule is that these key attributes must always contain a value; they cannot be left empty or set to NULL.

Understanding Entity Integrity

The constraint ensuring that no primary key attribute can have a NULL value is known as Entity Integrity. This rule guarantees that each row in a table represents a unique entity that can be identified.

• Primary Key Rule: Every table must have a primary key.
• Non-NULL Rule: The primary key column(s) cannot contain NULL values.

This constraint is crucial for maintaining the uniqueness and identify-ability of records within a database table.

Evaluating Other Options

The other constraints mentioned are distinct concepts:

• Referential Integrity: Focuses on maintaining valid relationships between tables using foreign keys. It ensures that foreign key values must match existing primary key values in the referenced table or be NULL (if allowed for the foreign key column itself).
• Functional Dependency: Describes a relationship where the value of one or more attributes determines the value of another attribute (e.g., a student ID determines the student's name).
• Multi-valued Dependency: A more complex dependency related to normalization, where an attribute determines a set of values for another attribute.

Therefore, the constraint preventing NULL values in key attributes is Entity Integrity.