Foreign Keys are great. For those of you who read some of my other posts, or heard me present on this material before, a foreign key can really help the Query Optimizer simplify out your query, because it knows that it must be able to find a match in an equality join (unless the field allows NULLs, which is a whole nother story).
I also blogged recently about the fact that a foreign key doesn't have to reference the primary key of a table. Turns out it can reference any candidate key, that is, one that has a unique index on it. This presents all kinds of interesting design ideas, such as avoiding joins by storing a different field (such as the username, rather than userid, if the username is the one thing you keep looking up to display at the top of the webpage). Referential integrity can still be maintained happily using an index seek (just a slightly larger index, but quick nonetheless), but the benefit could be huge if many queries no longer need to do join to get that info.
But I found an interesting question on Stack Overflow, which was asking about whether a foreign key could reference a view. He was trying to do it for the purpose of data integrity — which would probably require an expensive trigger. But let's talk about the Query Optimizer, which I think is another useful reason to have a foreign key hook into a view.
As a view is only a stored sub-query, it's a strange request, but one that is definitely interesting. A view can be instantiated as an indexed view, so I thought there was definitely potential in his question.
Unfortunately I had no luck trying to help him.
My first thought was "Well, you're going to need a unique index on the view — well that's doable — a Clustered Index on an Indexed View.", but you get an error saying that the foreign key needs to reference a user table. A user table? So it can't even reference a system table. But I guess that's fair enough these days, since we don't really have system tables any more (they're all system views, referencing underlying functions and the like).
My next thought was whether or not you could recreate something similar using a filtered index. After all, a filtered index satisfies many of the scenarios that indexed views often address. They can be unique for example, so you might use an indexed view to make sure that once a particular value is reached in a Status field, some other field must be unique. In SQL Server 2008 it's no problem to create a unique index and provide a WHERE clause for it. But unfortunately when you create a foreign key, you indicate the table you're referencing, not the index. I tried using a check constraint, to make sure that the foreign column could only contain values that were in the unique filtered index, but SQL just doesn't consider that a filtered index is available for referencing by a foreign key.
If you know a way to help this guy, follow the link and provide an answer. In a couple of days' time, the answer with the most votes will be automatically marked as the answer by the Stack Overflow system, which won't help him much. He understands that using views doesn't negate the FK constraints on the underlying tables, and I hope he can still work out a way of doing it without triggers, but at the end of the day, this other answer of mine, where I eventually say "Use triggers on the tables which contribute to your view" is probably the unfortunate truth.