Question

A database designer wants to ensure that no two customers can have the some Aadhaar Number in a table. Identify the SQL constraint suitable for this requirement.

• A. UNIQUE
• B. NOT NULL
• C. CHECK
• D. DEFAULT

Answer 1

Answer:

A. UNIQUE

SQL Constraint for Uniqueness

• The UNIQUE constraint is specifically designed to ensure that all values in a column are different, preventing duplicate entries across rows.
• When applied to an Aadhaar Number field, it ensures data integrity by enforcing that no two records share the same identifier, which is critical for identity verification systems.
• Key Characteristics of UNIQUE:
  1. It allows NULL values (unless a NOT NULL constraint is also explicitly applied).
  2. A table can have multiple UNIQUE constraints defined on different columns.
  3. It automatically creates a unique index on the specified column to optimize query performance during data retrieval and validation.
• Comparison with PRIMARY KEY:
  • A PRIMARY KEY constraint also enforces uniqueness but automatically implies NOT NULL and allows only one such key per table.
  • While a PRIMARY KEY is used to uniquely identify a row, the UNIQUE constraint is preferred for columns like Aadhaar Number, PAN, or Email that are unique but not necessarily the primary identifier of the record.