Meaning of NULL: Is a null value assigned when no other value applies (for example, the attribute is unknown, missing, or inapplicable)?

Introduction / Context:
NULL in relational databases represents the absence of a known, applicable value. It can mean unknown, missing, or inapplicable. Understanding NULL semantics is essential for correct querying, constraint design, and interpretation of results involving three-valued logic (true, false, unknown).

Given Data / Assumptions:

• NULL is not equal to any value, including another NULL.
• NULL can appear in any nullable column type, not just numeric or text.
• Constraints such as NOT NULL or foreign key requirements may restrict NULL usage.

Concept / Approach:
The statement says a null value is assigned when no other value applies. That captures a core use case: inapplicability. It also aligns with unknown or missing situations, in which a proper value is not presently available. Thus, using NULL indicates the lack of a concrete value rather than a specific default.

Step-by-Step Solution:
Determine attribute optionality; declare columns NULL or NOT NULL accordingly.When data is missing or inapplicable, store NULL in nullable columns.Use IS NULL / IS NOT NULL in predicates to test for NULLs.Be mindful that comparisons with NULL yield unknown; use COALESCE when appropriate.Ensure aggregates and joins handle NULLs as intended (e.g., COUNT(*) vs. COUNT(column)).

Verification / Alternative check:
Run sample queries showing differences between = NULL (wrong) and IS NULL (correct), and observe how aggregates skip NULLs for COUNT(column).

Why Other Options Are Wrong:
Incorrect: contradicts the established meaning of NULL.Only numeric or only optional FKs: NULL usage applies broadly across data types and constraints, subject to column definitions.

Common Pitfalls:
Using sentinel values (like 0 or 'unknown') instead of NULL; forgetting three-valued logic; assuming NULL equals empty string, which is DBMS-dependent and generally incorrect.

Final Answer:
Correct