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

-- Now we’re run our conditional CREATE statement
IF NOT EXISTS (
SELECT s.name AS SchemaName, t.name AS TableName
FROM sys.schemas s
JOIN sys.tables t
ON s.schema_id = t.schema_id
WHERE s.name = ‘dbo’
AND t.name = ‘OurIFTest’
)
CREATE TABLE OurIFTest(
Col1 int PRIMARY KEY
);
-- And now look again to prove that it’s been created.
SELECT ‘Found Table ‘ + s.name + ‘.’ + t.name
FROM sys.schemas s
JOIN sys.tables t
ON s.schema_id = t.schema_id
WHERE s.name = ‘dbo’
AND t.name = ‘OurIFTest’;
The meat of this is in the middle — notice that our CREATE TABLE statement runs only if no matching table
already exists. The first check we did on it (right at the beginning of the script) found the table didn’t exist,
so we know that the IF is going to be true and our CREATE TABLE will execute.
-----------------------------------------------------------------------------
(0 row(s) affected)
-----------------------------------------------------------------------------Found
Table dbo.OurIFTest
(1 row(s) affected)
The ELSE Clause
Chapter 11: Writing Scripts and Batches
Now this thing about being able to run statements conditionally is just great, but it doesn’t really deal
with all the scenarios we might want to deal with. Quite often — indeed, most of the time — when we
deal with an IF condition, we have specific statements we want to execute not just for the true condition,
but also a separate set of statements that we want to run if the condition is false — or the ELSE
condition.
You will run into situations where a Boolean cannot be evaluated — that is, the
result is unknown (for example, if you are comparing to a NULL). Any expression
that returns a result that would be considered as an unknown result will be treated
as FALSE.
The ELSE statement works pretty much as it does in any other language. The exact syntax may vary
slightly, but the nuts and bolts are still the same; the statements in the ELSE clause are executed if the
statements in the IF clause are not.
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