What is a primary key vs a unique key?

What is a primary key vs a unique key?

Approach: Compare constraints and count per table.

A primary key uniquely identifies a row and cannot be NULL; there is only one per table. A unique key also uniquely identifies a row but can allow NULL values; there can be multiple per table.

Follow-up: Can a unique key have multiple NULL values?

Key concepts: null values, table constraint.