Primary Key vs Unique Key in DBMS: Key Differences Explained
A Primary Key is the column (or set of columns) that uniquely identifies every row and never allows NULLs. A Unique Key also enforces uniqueness, but accepts one NULL and can coexist with other Unique Keys.
People confuse them because both say “unique,” and in quick sketches we often draw only one. Yet when you add an email field, the Primary Key stays the ID, while the email becomes a Unique Key—real life needs both.
Key Differences
Primary Key: one per table, NOT NULL, default clustered index. Unique Key: many allowed, one NULL permitted, optional non-clustered index. Foreign keys can reference either, but most designers target the Primary Key for clarity.
Which One Should You Choose?
Use a Primary Key for the row’s absolute identity—think order_id. Add a Unique Key whenever another column must also stay unique—like username or passport_number—without replacing the main identifier.
Examples and Daily Life
Imagine a library app. ISBN is the Primary Key for books. Email is a Unique Key on the patrons table—two users can’t share it, yet one may leave email blank.
Can a table have multiple Primary Keys?
No. One composite Primary Key is allowed, but not several separate ones.
Does Unique Key stop duplicate NULLs?
Standards allow one NULL; most databases treat all NULLs as distinct, so only one NULL row is accepted.
Can I change a Primary Key later?
Yes, but it drops and rebuilds indexes, so plan carefully in production.