In relational databases, which of the following represents an absent value that may exist but is unknown or does not exist at all?

Correct option is C

A null value in relational databases represents an unknown or missing value. It indicates that the value of a field is either not applicable, unknown, or deliberately left blank. Unlike zero or empty strings, a null value specifically conveys the absence of any value, making it an essential concept in database design and integrity.
Important Key Points:
1. Definition of Null Value: A null value means "unknown" or "not applicable" and is distinct from zero or an empty string.
2. Database Operations: Null values can impact operations like comparisons and aggregations, requiring special handling (e.g., SQL's IS NULL or IS NOT NULL conditions).
3. Integrity Maintenance: Nulls allow databases to maintain integrity when a value is legitimately absent or unknown.
Knowledge Booster:
· Empty Table: Refers to a database table with no rows, unrelated to the concept of null values.
· Placeholder Value: A temporary or default value used as a substitute until the actual value is provided.
· Default Value: A predefined value assigned to a field when no explicit value is provided, unlike null, which indicates no value.
· Zero Value: A valid numeric value representing "0," which is different from null as it implies the existence of a defined value.