Integrity management in SQL: Which command is used to enable, disable, or drop an integrity constraint defined on an existing table?

Introduction / Context: Integrity constraints (PRIMARY KEY, UNIQUE, FOREIGN KEY, CHECK) enforce data correctness and relationships. After deployment, administrators may need to enable, disable, or drop a constraint temporarily (for bulk loads) or permanently (during schema changes). Selecting the correct command is essential for precise control with minimal disruption.

Given Data / Assumptions:

• We are operating on an existing table.
• The task involves constraint state or definition (enable/disable/drop).
• Standard SQL semantics are assumed; vendor specifics may add options.

Concept / Approach: The correct command for managing constraints on an existing table is ALTER TABLE. It supports adding, modifying, enabling/disabling (vendor-specific), and dropping constraints. Non-standard verbs like “DEFINE TABLE” or “MODIFY TABLE” are not part of portable SQL syntax and should be avoided unless referencing a specific vendor feature.

Step-by-Step Solution:

Verification / Alternative check: Vendor manuals document ALTER TABLE as the entry point for constraint lifecycle management; some vendors support ENABLE/DISABLE keywords or require drop/recreate sequences.

Why Other Options Are Wrong:

• DEFINE TABLE and MODIFY TABLE: not standard SQL commands.
• All of the above: incorrect because only ALTER TABLE is standard and broadly correct.

Common Pitfalls: Disabling constraints without revalidating data later; dropping constraints and forgetting to recreate them with the same semantics.

Final Answer: ALTER TABLE