What are views good for?

问题:

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?
   • Are there any situations in which it is tempting to use a view when you shouldn't use one?
   • Why would you use a view in lieu of something like a table-valued function or vice versa?
   • 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.)

回答1:

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 ;)

回答2:

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.

回答3:

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.

回答4:

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.

回答5:

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.

回答6:

In addition to what the others have stated, views can also be useful for removing more complecated SQL queries from the application.

As an example, instead of in an application doing:

sql = "select a, b from table1 union select a, b from table2";

You could abstract that to a view:

create view union_table1_table2_v as
select a,b from table1
union
select a,b from table2

and in the app code, simply have:

sql = "select a, b from union_table1_table2_v";

Also if the data structures ever change, you won't have to change the app code, recompile, and redeploy. you would just change the view in the db.

回答7:

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.

回答8:

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.

回答9:

The responses so far are correct -- views are good for providing security, denormalization (although there is much pain down that road if done wrong), data model abstraction, etc.

In addition, views are commonly used to implement business logic (a lapsed user is a user who has not logged in in the last 40 days, that sort of thing).

回答10:

Views save a lot of repeated complex JOIN statements in your SQL scripts. You can just encapsulate some complex JOIN in some view and call it in your SELECT statement whenever needed. This would sometimes be handy, straight forward and easier than writing out the join statements in every query.

回答11:

A view is simply a stored, named SELECT statement. Think of views like library functions.

回答12:

I wanted to highlight the use of views for reporting. Often, there is a conflict between normalizing the database tables to speed up performance, especially for editing and inserting data (OLTP uses), and denormalizing to reduce the number of table joins for queries for reporting and analysis (OLAP uses). Of necessity, OLTP usually wins, because data entry must have optimal performance. Creating views, then, for optimal reporting performance, can help to satisfy both classes of users (data entry and report viewers).

回答13:

I remember a very long SELECT which involved several UNIONs. Each UNION included a join to a price table which was created on the fly by a SELECT that was itself fairly long and hard to understand. I think it would have been a good idea to have a view that to create the price table. It would have shortened the overall SELECT by about half.

I don't know if the DB would evaluate the view once, or once each time in was invoked. Anyone know? If the former, using a view would improved performance.

回答14:

Anytime you need [my_interface] != [user_interface].

Example:

TABLE A:

• id
• info

VIEW for TABLE A:

• Customer Information

this is a way you might hide the id from the customer and rename the info to a more verbose name both at once.

The view will use underlying index for primary key id, so you won't see a performance loss, just better abstraction of the select query.