Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

It’s surprising to many, but you can run INSERT, UPDATE, and DELETE statements against a view successfully.
There are several things, however, that you need to keep in mind when changing data through a view:
❑ If the view contains a join, you won’t, in most cases, be able to INSERT or DELETE data unless
you make use of an INSTEAD OF trigger. An UPDATE can, in some cases (as long as you are only
updating columns that are sourced from a single table), work without INSTEAD OF triggers, but
it requires some planning, or you’ll bump into problems very quickly.
❑ If your view references only a single table, then you can INSERT data using a view without the
use of an INSTEAD OF trigger provided all the required fields in the table are exposed in the
view or have defaults. Even for single table views, if there is a column not represented in the
view that does not have a default value, then you must use an INSTEAD OF trigger if you want
to allow an INSERT.
❑ You can, to a limited extent, restrict what is and isn’t inserted or updated in a view.
Now — I’ve already mentioned INSTEAD OF triggers several times. The problem here is the complexity
of INSTEAD OF triggers and that we haven’t discussed triggers to any significant extent yet. As is often
the case in SQL Server items, we have something of the old chicken vs. egg thing going (“Which came
first?”). I need to discuss INSTEAD OF triggers because of their relevance to views, but we’re also not
ready to talk about INSTEAD OF triggers unless we understand both of the objects (tables and views) that
they can be created against.
The way we are going to handle things for this chapter is to address views the way they used to be —
before there was such a thing as INSTEAD OF triggers. While we won’t deal with the specifics of INSTEAD
OF triggers in this chapter, we’ll make sure we understand when they must be used. We’ll then come
back and address these issues more fully when we look briefly at INSTEAD OF triggers in Chapter 15.
Having said that, I will provide this bit of context — An INSTEAD OF trigger is a special kind of trigger
that essentially runs “instead” of whatever statement caused the trigger to fire. The result is that it
can see what your statement would have done, and then make decisions right in the trigger about how
to resolve any conflicts or other issues that might have come up. It’s very powerful, but also fairly complex
stuff, which is why we defer it for now.
Dealing with Changes in Views with Joined Data
If the view has more than one table, then using a view to modify data is, in many cases, out — sort of
anyway — unless you use an INSTEAD OF trigger (more on this in a moment). Since it creates some
ambiguities in the key arrangements, Microsoft locks you out by default when there are multiple tables.
To resolve this, you can use an INSTEAD OF trigger to examine the altered data and explicitly tell SQL
Server what you want to do with it.
Required Fields Must Appear in the View or Have Default Value
Chapter 10: Views
By default, if you are using a view to insert data (there must be a single table SELECT in the underlying
query or at least you must limit the insert to affect just one table and have all required columns represented),
then you must be able to supply some value for all required fields (fields that don’t allow
NULLs). Note that by “supply some value” I don’t mean that it has to be in the SELECT list — a default
covers the bill rather nicely. Just be aware that any columns that do not have defaults and do not accept
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