Craig Farrell (10/13/2010)
In SQL Server a unique index on non-nullable columns is just a key by another name. Unique indexes and uniqueness constraints are simply two different ways to create keys. I would not recommend using a unique index instead of a uniqueness constraint. Constraints, not indexes, are the conventional, SQL standard and most obvious way to implement keys.
Same difference.
No it's not the same for the reasons I already stated: Constraints are standard SQL, unique indexes are not; Constraints are understood and used by more people; Some software tools will recognise keys declared as constraints but will not recognise those created using the CREATE INDEX syntax. In my view those are all very good reasons to use uniquness constraints and avoid creating unique indexes directly.
Thus, while a unique can be an alternate key, an alternate key is always unique. The difference is not inconsequential, thus I separate the concept. The fact that some data is usually mobile in a 'natural key', say, like the company's name, is why I like surrogate keys, like identity fields. This means that if a user edits the company's name (which is unique, at least by state), we still haven't lost our unique locator that everything else hooks to it with.
That's more or less what I thought you meant, but it's not technically correct. Two things make a key (aka "candidate key") and they are 1) uniqueness and 2) irreducibility. Those are the two fundamental qualifications for a key. Stability is usually desirable as well but the fact that a key changes does not make it any less of a key than some other key that doesn't change. Of course that's just terminology I suppose, the effect is exactly the same whether you choose to call it a key or not - the same considerations apply. But I think it's worth saying because a lot of people do seem to get very confused about what keys are and why they are important.
I'm avoiding the term "alternate key" because primary and alternate keys are really the same thing. All are candidate keys.