Question

In almost each code example by creating mysql joinUs table, there is the code like this:

CREATE TABLE `members` (
`id` int(10) unsigned NOT NULL auto_increment,
`username` text, //unique
`email` text, //unique
`pass` text,
PRIMARY KEY  (`id`)

So, if username and/or email is set as unique, what is then the purpose of id field?
Can I simply set username as primary key and exclude the id from the table?

Answer 1

Id is a nice convention because it will never change. That lets other tables reference users via their id, and enables you to let users change their usernames and emails. Having said that, yes, you could use username as a primary key.