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

Let’s look at the specific syntax:
AS
The first item is a little different; it provides a column name to go with the value. This is simply the alias
that we’re going to use to refer to the value that is computed, based on the expression that follows the AS
keyword.
Next comes the computed column expression. The expression can be any normal expression that uses
either literals or column values from the same tables. Therefore, in our example of price and quantity, we
might define this column as:
ExtendedPrice AS Price * Quantity
For an example using a literal, let’s say that we always charge a fixed markup on our goods that is 20
percent over our cost. We could simply keep track of cost in one column, and then use a computed column
for the ListPrice column:
ListPrice AS Cost * 1.2
Pretty easy, eh? There are a few caveats and provisos though:
Chapter 5: Creating and Altering Tables
❑ You cannot use a subquery, and the values cannot come from a different table.
❑ In SQL Server 2000 and earlier, you could not use a computed column as any part of any key
(primary, foreign, or unique) or with a default constraint. For SQL Server 2005, you can now use
a computed column in constraints (you must flag the computed column as persisted if you do
this, however).
❑ Special steps must be taken if you want to create indexes on computed columns. We will discuss
these steps when we explore indexing in Chapter 9.
We’ll look at specific examples of how to use computed columns a little later in this chapter.
Even years after they were added to the product, I’m still rather surprised that I don’t hear much debate
about the use of computed columns. Rules for normalization of data say that we should not have a column
in our table for information that can be derived from other columns — that’s exactly what a computed
column is!
I’m glad the religious zealots of normalization haven’t weighed into this one much, as I like computed
columns as something of a compromise. By default, you aren’t storing the data twice, and you don’t
have issues with the derived values not agreeing with the base values because they are calculated on the
fly directly from the base values. However, you still get the end result you wanted. Note that, if you
index the computed column, you are indeed actually storing the data (you have to for the index). This,
however, has its own benefits when it comes to read performance.
This isn’t the way to do everything related to derived data, but it sure is an excellent helper for many
situations.
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