What are views good for?

Question

I'm just trying to get a general idea of what views are used for in RDBMSes. That is to say, I know what a view is and how to make one. I also know what I've used them for in the past.

But I want to make sure I have a thorough understanding of what a view is useful for and what a view shouldn't be useful for. More specifically:

1. What is a view useful for?
2. Are there any situations in which it is tempting to use a view when you shouldn't use one?
3. Why would you use a view in lieu of something like a table-valued function or vice versa?
4. Are there any circumstances that a view might be useful that aren't apparent at first glance?

(And for the record, some of these questions are intentionally naive. This is partly a concept check.)

Answer 1

In a way, a view is like an interface. You can change the underlying table structure all you want, but the view gives a way for the code to not have to change.

Views are a nice way of providing something simple to report writers. If your business users want to access the data from something like Crystal Reports, you can give them some views in their account that simplify the data -- maybe even denormalize it for them.

Answer 2

1) What is a view useful for?

IOPO In One Place Only

•Whether you consider the data itself or the queries that reference the joined tables, utilizing a view avoids unnecessary redundancy.

•Views also provide an abstracting layer preventing direct access to the tables (and the resulting handcuffing referencing physical dependencies). In fact, I think it's good practice¹ to offer only abstracted access to your underlying data (using views & table-valued functions), including views such as

CREATE VIEW AS
      SELECT * FROM tblData

¹I hafta admit there's a good deal of "Do as I say; not as I do" in that advice ;)

2) Are there any situations in which it is tempting to use a view when you shouldn't use one?

Performance in view joins used to be a concern (e.g. SQL 2000). I'm no expert, but I haven't worried about it in a while. (Nor can I think of where I'm presently using view joins.)

Another situation where a view might be overkill is when the view is only referenced from one calling location and a derived table could be used instead. Just like an anonymous type is preferable to a class in .NET if the anonymous type is only used/referenced once.

    • See the derived table description in   http://msdn.microsoft.com/en-us/library/ms177634.aspx

3) Why would you use a view in lieu of something like a table-valued function or vice versa?

(Aside from performance reasons) A table-valued function is functionally equivalent to a parameterized view. In fact, a common simple table-valued function use case is simply to add a WHERE clause filter to an already existing view in a single object.

4) Are there any circumstances that a view might be useful that aren't apparent at first glance?

I can't think of any non-apparent uses of the top of my head. (I suppose if I could, that would make them apparent ;)