主键应该选择一个静态属性。由于电子邮件地址不是静态的，可以被多个候选人共享，因此使用它们作为主键并不是一个好主意。此外，电子邮件地址通常是一定长度的字符串，可能大于唯一id，我们想使用[len(email_address)>len(unique_id)]，所以它将需要更多的空间，甚至最糟糕的是，它们被多次存储为外键。因此会导致性能下降。

如果只是要求电子邮件是唯一的，那么你可以用该列创建一个唯一的索引。

似乎没有人提到一个可能的问题，即电子邮件地址可能被视为隐私。如果电子邮件地址是主键，那么个人资料页面的URL很可能类似于..../Users/my@email.com。如果不想暴露用户的电子邮件地址怎么办?您必须找到其他一些识别用户的方法，可能是通过一个唯一的整数值来生成像..../Users/1这样的url。那么你最终会得到一个唯一的整数值。

你的同事是对的:使用一个自动递增的整数作为你的主键。

您可以在应用程序级别实现电子邮件唯一性，或者您可以将电子邮件地址列标记为惟一，并在该列上添加索引。

将字段添加为唯一字段只会在插入到该表时花费字符串比较的成本，而不会在执行连接和外键约束检查时花费字符串比较成本。

当然，您必须注意，在数据库级别向应用程序添加任何约束都可能导致应用程序变得不灵活。在仅仅因为应用程序需要某个字段是唯一的或非空的，就将它设置为“唯一的”或“非空的”之前，一定要充分考虑。

使用电子邮件地址作为主键的缺点:

Slower when doing joins. Any other record with a posted foreign key now has a larger value, taking up more disk space. (Given the cost of disk space today, this is probably a trivial issue, except to the extent that the record now takes longer to read. See #1.) An email address could change, which forces all records using this as a foreign key to be updated. As email address don't change all that often, the performance problem is probably minor. The bigger problem is that you have to make sure to provide for it. If you have to write the code, this is more work and introduces the possibility of bugs. If your database engine supports "on update cascade", it's a minor issue.

使用电邮地址作主键的优点:

You may be able to completely eliminate some joins. If all you need from the "master record" is the email address, then with an abstract integer key you would have to do a join to retrieve it. If the key is the email address, then you already have it and the join is unnecessary. Whether this helps you any depends on how often this situation comes up. When you are doing ad hoc queries, it's easy for a human being to see what master record is being referenced. This can be a big help when trying to track down data problems. You almost certainly will need an index on the email address anyway, so making it the primary key eliminates one index, thus improving the performance of inserts as they now have only one index to update instead of two.

在我看来，这两种情况都不是十拿九稳的。当有实用的键时，我倾向于使用自然键，因为它们更容易使用，而且在大多数情况下，缺点并不太重要。

不要使用电子邮件地址为主键，保持电子邮件的唯一性，但不要使用它为主键，使用用户id或用户名为主键