Oracle Primary Key vs Unique Key: Key Differences Explained
Oracle Primary Key is a column (or set) that uniquely identifies each row and forbids NULLs. Unique Key also enforces uniqueness but allows one NULL per column, making it a lighter, optional constraint.
Developers often slap a Primary Key on every table because ORMs demand it, then add extra Unique Keys for business rules like “email must be unique.” The mix-up happens when the same natural key appears in both places, causing debates about which is “more correct.”
Key Differences
Only one Primary Key per table, never accepts NULLs, and automatically gets a backing index. Multiple Unique Keys are allowed, each tolerating one NULL, and you can disable or defer them without dropping the table.
Which One Should You Choose?
Use Primary Key for the row’s true identifier—surrogate or composite. Reach for Unique Key when an alternate business key must stay distinct but isn’t the main address of the row. Keep them separate; duplicating the same columns wastes space and confuses optimizers.
Examples and Daily Life
A banking app sets ACCOUNT_ID as Primary Key (no NULLs) and EMAIL as Unique Key (one NULL allowed). Later, a promo code column gets another Unique Key so users can’t reuse the same code twice.
Can I switch a Unique Key to a Primary Key later?
Yes, but you must first drop the Unique constraint and ensure the column has no NULLs before adding the Primary Key.
Does a Unique Key speed up queries like a Primary Key?
It creates an index, so it helps lookups, yet foreign keys usually reference the Primary Key for optimal joins.
What happens if both keys cover the same column?
Oracle keeps two identical indexes, doubling maintenance cost; choose one or keep them on different column sets.