What does a Unique Constraint enforce?

What does a Unique Constraint enforce?

• A. Forces each value of a column or table to be unique
• B. Ensures data type consistency across the table
• C. Prevents deletion of a referenced row
• D. Creates an index automatically

Answer: (A)

Unique constraints ensure that the values in a column or in a combination of columns are distinct across all rows in the table. This prevents duplicates, which is essential for identifiers like emails or usernames. You can apply it to a single column or to multiple columns to enforce the uniqueness of the tuple across the table. In many systems, the enforcement is implemented via a unique index behind the scenes, but the main idea is to maintain data integrity by disallowing duplicate values. It doesn’t govern data types, nor does it control deletions of referenced rows—that’s handled by data type definitions and foreign key behavior, respectively. While an underlying index may be created to support the constraint, the core purpose is the guarantee of uniqueness.

Unique constraints ensure that the values in a column or in a combination of columns are distinct across all rows in the table. This prevents duplicates, which is essential for identifiers like emails or usernames. You can apply it to a single column or to multiple columns to enforce the uniqueness of the tuple across the table. In many systems, the enforcement is implemented via a unique index behind the scenes, but the main idea is to maintain data integrity by disallowing duplicate values. It doesn’t govern data types, nor does it control deletions of referenced rows—that’s handled by data type definitions and foreign key behavior, respectively. While an underlying index may be created to support the constraint, the core purpose is the guarantee of uniqueness.