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 f

Answer 1

Views can centralize or consolidate data. Where I'm at we have a number of different databases on a couple different linked servers. Each database holds data for a different application. A couple of those databases hold information that are relavent to a number of different applications. What we'll do in those circumstances is create a view in that application's database that just pulls data from the database where the data is really stored, so that the queries we write don't look like they're going across different databases.

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 ;)

Answer 3

In a sense views denormalize. Denormalization is sometimes necessary to provide data in a more meaningful manner. This is what a lot of applications do anyway by way of domain modeling in their objects. They help present the data in a way that more closely matches a business' perspective.

Answer 4

Views hide the database complexity. They are great for a lot of reasons and are useful in a lot of situations, but if you have users that are allowed to write their own queries and reports, you can use them as a safeguard to make sure they don't submit badly designed queries with nasty cartesian joins that take down your database server.

Answer 5

The OP asked if there were situations where it might be tempting to use a view, but it's not appropriate.

What you don't want to use a view for is a substitute for complex joins. That is, don't let your procedural programming habit of breaking a problem down into smaller pieces lead you toward using several views joined together instead of one larger join. Doing so will kill the database engine's efficiency since it's essentially doing several separate queries rather than one larger one.

For example, let's say you have to join tables A, B, C, and D together. You may be tempted to make a view out of tables A & B and a view out of C & D, then join the two views together. It's much better to just join A, B, C, and D in one query.

Answer 6

Views can be used to provide security (ie: users can have access to views that only access certain columns in a table), views can provide additional security for updates, inserts, etc. Views also provide a way to alias column names (as do sp's) but views are more of an isolation from the actual table.