Which constraint best enforces unique business rules, such as a unique email per user?

Which constraint best enforces unique business rules, such as a unique email per user?

• A. Check constraint
• B. Not null constraint
• C. Default value constraint
• D. Unique constraint

Answer: (D)

Ensuring data integrity often means putting rules in place at the database level. For a business rule like “each user has a unique email,” you need a constraint that prevents two rows from sharing the same email value. A unique constraint enforces that all values in the email column are distinct across the table, so attempts to insert or update a record with a duplicate email are rejected. This directly supports the one-user-one-email rule. Other constraints serve different purposes. A not null constraint makes sure an email is provided, but it doesn’t prevent duplicates. A default value fills in a value when none is supplied, yet doesn’t ensure uniqueness. A check constraint enforces a condition or pattern (for example, a format) but by itself doesn’t guarantee that two users won’t share the same email unless a uniqueness guarantee is also present. So, the constraint that best enforces unique business rules like a unique email per user is the unique constraint.

Ensuring data integrity often means putting rules in place at the database level. For a business rule like “each user has a unique email,” you need a constraint that prevents two rows from sharing the same email value. A unique constraint enforces that all values in the email column are distinct across the table, so attempts to insert or update a record with a duplicate email are rejected. This directly supports the one-user-one-email rule.

Other constraints serve different purposes. A not null constraint makes sure an email is provided, but it doesn’t prevent duplicates. A default value fills in a value when none is supplied, yet doesn’t ensure uniqueness. A check constraint enforces a condition or pattern (for example, a format) but by itself doesn’t guarantee that two users won’t share the same email unless a uniqueness guarantee is also present.

So, the constraint that best enforces unique business rules like a unique email per user is the unique constraint.