Question

explain why using email address as the primary key for Staff is a bad idea

Answer 1

There are multiple reasons why the e-mail addresses being primary keys are a bad idea. I'm listing some below:

1. Primary keys are supposed to be UNIQUE and CONSTANT. E-mail addresses are liable to change. They cannot be changed and hence, it's inconvenient. This might happen if some e-mail provider goes out of business and now you have to update all the user's e-mails and the fields associated with the primary key.
2. It will be slower when doing joins.
3. Any other record that has a posted foreign key will have a larger value now, taking up more disk space. (It might be a trivial issue but the record now takes longer to read.)
4. String comparisons are generally slower than integer comparisons. It might not be an issue if you're looking up a user but it will be a problem if there are complex joins.
5. Some businesses might share a common e-mail service and this will lead to duplicate fields. For eg., kbiswal@some_company.com might belong to both Kamal Biswal and Kritesh Biswal.

This is why an e-mail might be useful as a secondary key for a lookup but using it as a primary key will be very inconvenient in the long run.