Default is a constraint or a feature

In RDBMS, is "Default" a constraint or just a feature? While most books classify it as a constraint, it functions more like a feature. Here’s why:

Unlike constraints such as "NOT NULL," which prevents the insertion or updating of null values, or "Primary Key," which enforces uniqueness and non-nullability, or "Foreign Key," which ensures referential integrity, the "Default" clause does not enforce any business rule or data validation.

Other constraints actively validate and enforce rules during both insert and update operations:

• NOT NULL: Prevents null values.
• Primary Key: Ensures uniqueness and non-nullability.
• Unique Key: Restricts duplicate values.
• Foreign Key: Enforces references to valid values in a related table.
• Check Constraint: Allows custom validations for specific conditions.

In contrast, the Default clause only applies at the time of insertion and provides a fallback value when no explicit value is provided. It does not validate or enforce correctness in the same way as other constraints.

What do you think about this? Please share your comments!