Email addresses are awful primary keys. Sure, it's unique. But the other main property of a good primary key is that it never changes. Email addresses change co

Email addresses are awful primary keys. Sure, it's unique. But the other main property of a good primary key is that it never changes. Email addresses change constantly. People use their work or university address for personal things, and then change jobs / are laid off / graduate. Or they use an email from their ISP, and move. Or they used to use one webmail system, and switch to one with a better UI; buy their first Apple device switch to an icloud email instead of a gmail one. It's fine to use an email address as the thing you enter into a login box. But internally you really want the user records to be keyed by something else, and just associate an arbitrary number of email addresses to that.