Which constraint guarantees uniqueness and non-null values for a set of columns?

Which constraint guarantees uniqueness and non-null values for a set of columns?

• A. Unique Constraint
• B. Not Null Constraint
• C. Primary Key Constraint
• D. Check Constraint

Answer: (C)

The primary key constraint is designed to identify each row uniquely and to ensure those identifying values are never null. When you declare a primary key on one or more columns, the database enforces two things: (1) each row must have a unique value in those columns, and (2) none of those values can be NULL. This combination lets the key reliably identify every row and serves as a natural target for foreign keys in other tables. It can be a single column or a set of columns (composite primary key), which means you can enforce uniqueness across multiple fields as needed. Other constraints cover only one aspect things: a Unique constraint enforces uniqueness (often with NULLs allowed, depending on the system), a Not Null constraint forbids NULLs but doesn’t guarantee uniqueness, and a Check constraint enforces a rule on values but doesn’t inherently guarantee a unique, non-null key.

The primary key constraint is designed to identify each row uniquely and to ensure those identifying values are never null. When you declare a primary key on one or more columns, the database enforces two things: (1) each row must have a unique value in those columns, and (2) none of those values can be NULL. This combination lets the key reliably identify every row and serves as a natural target for foreign keys in other tables. It can be a single column or a set of columns (composite primary key), which means you can enforce uniqueness across multiple fields as needed. Other constraints cover only one aspect things: a Unique constraint enforces uniqueness (often with NULLs allowed, depending on the system), a Not Null constraint forbids NULLs but doesn’t guarantee uniqueness, and a Check constraint enforces a rule on values but doesn’t inherently guarantee a unique, non-null key.