Primary Key vs. Unique Key: Core Differences Explained
A Primary Key is the single, non-null column (or set of columns) that uniquely identifies every row and serves as the table’s main reference; a Unique Key also enforces uniqueness but can allow one NULL and is optional, not the main anchor.
People mix them up because many tutorials show both stopping duplicates, so devs think “unique = primary.” Picture a café loyalty app: CustomerID is the Primary Key for every order, while Email is a Unique Key to stop duplicate accounts yet still allow guests without an email.
Key Differences
Primary Key: exactly one per table, never NULL, clustered by default. Unique Key: many allowed, one NULL per column, non-clustered unless forced. Foreign keys always point to Primary, not Unique.
Which One Should You Choose?
Use a Primary Key for every table—an auto-increment ID is safest. Add Unique Keys only when another column must stay duplicate-free, like Social Security Number or SKU code.
Examples and Daily Life
Think of a student database: StudentID (Primary Key) drives grades, schedules, payments. Meanwhile, PassportNumber is set as a Unique Key so no two students share the same passport, yet exchange students without passports can still enroll (NULL allowed).
Can a table exist without a Primary Key?
Yes, but it’s bad practice; most ORMs, frameworks, and replication tools expect one for reliable row addressing.
Is a Unique Key automatically an index?
Yes. Declaring a Unique Key creates a unique index behind the scenes, speeding up look-ups while enforcing the no-duplicates rule.
Can a Primary Key span multiple columns?
Absolutely. A composite Primary Key like (FlightNumber, SeatNumber) ensures each seat on each flight is uniquely identified.