Note also that "key" does not necessarily mean primary key. Something like this will work:
CREATE TABLE book (
isbn BIGINT NOT NULL PRIMARY KEY,
id INT NOT NULL AUTO_INCREMENT,
accepted_terms BIT(1) NOT NULL,
accepted_privacy BIT(1) NOT NULL,
INDEX(id)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
This is a contrived example and probably not the best idea, but it can be very useful in certain cases.