Both ensure uniqueness, but with key differences:

PRIMARY KEY:

• Cannot contain NULL values
• Only one per table
• Automatically creates a unique index
• Used for table relationships

UNIQUE:

• Can contain NULL values (multiple NULLs allowed)
• Multiple unique constraints per table
• Creates a unique index

CREATE TABLE users (
    id SERIAL PRIMARY KEY,        -- Cannot be NULL
    email VARCHAR(255) UNIQUE,    -- Can have one NULL
    username VARCHAR(50) UNIQUE   -- Multiple unique constraints allowed
);