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

IF NOT EXISTS
(SELECT ‘True’
FROM sys.databases
WHERE name = ‘AdventureWorksCreate’)
BEGIN
CREATE DATABASE AdventureWorksCreate;
END
ELSE
BEGIN
PRINT ‘Database already exists. Skipping CREATE DATABASE Statement’;
END
GO
The first time you run this, there won’t be any database called AdventureWorksCreate (unless by sheer
coincidence you created something called that before we got to this point), so you’ll get a response that
looks like this:
Command(s) completed successfully.
This was unhelpful in terms of telling you what exactly was done, but at least you know it thinks it did
what you asked.
Now run the script a second time and you’ll see a change:
Database already exists. Skipping CREATE DATABASE Statement
Chapter 7: Adding More to Our Queries
So, without much fanfare or fuss, we’ve added a rather small script that will make things much more
usable for the installers of your product. That may be an end user who bought your off-the-shelf product,
or it may be you — in which case it’s even better that it’s fully scripted.
The long and the short of it is that EXISTS is a very handy keyword indeed. It can make some queries
run much faster, and it can also simplify some queries and scripts.
A word of caution here — this is another one of those places where it’s easy to get trapped in “traditional
thinking.” While EXISTS blows other options away in a large percentage of queries where
EXISTS is a valid construct, that’s not always the case. For example, the query used as a derived table
example can also be written with a couple of EXISTS operators (one for each product), but the derived
table happens to run more than twice as fast. That’s definitely the exception, not the rule — EXISTS
will normally smoke a derived table for performance. Just remember that rules are sometimes made to be
broken.
Mixing Data T ypes: CAST and CONVERT
You’ll see both CAST and CONVERT used frequently. Indeed, we’ve touched briefly on both of these
already in this chapter. Considering how often we’ll use these two functions, this seems like a good time
to look a little closer at what they can do for you.
Both CAST and CONVERT perform data-type conversions for you. In most respects, they both do the same
thing, with the exception that CONVERT also does some date-formatting conversions that CAST doesn’t offer.
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