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

Chapter 7: Adding More to Our Queries
that it knows to return FALSE for that lookup rather than TRUE. Performance-wise, everything else about
the query is the same.
Using EXISTS in Other Ways
202
One common use of EXISTS is to check for the existence of a table before running a CREATE statement.
You may want to drop an existing table, or you just may way to change to an ALTER statement or some
other statement that adjusts the existing table if there is one. One of the most common ways you’ll see
this done will look something like this:
IF EXISTS
(SELECT *
FROM sys.objects
WHERE OBJECT_NAME(object_id) = 'foo'
AND SCHEMA_NAME(schema_id) = 'dbo'
AND OBJECTPROPERTY(object_id, 'IsUserTable') = 1)
BEGIN
DROP TABLE dbo.foo;
PRINT 'Table foo has been dropped';
END
GO
CREATE TABLE dbo.foo
(
Column1 int IDENTITY(1,1) NOT NULL,
Column2 varchar(50)NULL
);
Since EXISTS returns nothing but TRUE or FALSE, that means it works as an excellent conditional expression.
The preceding example will run the DROP TABLE code only if the table exists; otherwise, it skips over
that part and moves right into the CREATE statement. This avoids one of two errors showing up when you
run the script. First, that it can’t run the CREATE statement (which would probably create other problems
if you were running this in a script where other tables were depending on this being done first) because
the object already exists. Second, that it couldn’t DROP the table (this pretty much just creates a message
that might be confusing to a customer who installs your product) because it didn’t exist. You’re covered
for both.
As an example of this, let’s write our own CREATE script for something that’s often skipped in the automation
effort — the database. But creation of the database is often left as part of some cryptic directions that
say something like “create a database called ‘xxxx’.” The fun part is when the people who are actually
installing it (who often don’t know what they’re doing) start including the quotes, or create a database
that is too small, or a host of other possible and very simple errors to make. This is the point where I hope
you have a good tech support department.
Instead, we’ll just build a little script to create the database object that could go with AdventureWorks2008.
For safety’s sake, we’ll call it AdventureWorksCreate. We’ll also keep the statement to a minimum because
we’re interested in the EXISTS rather than the CREATE command:
USE MASTER;
GO