What is the difference between PRIMARY KEY and UNIQUE constraints?

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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
);

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