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

Chapter 11: Writing Scripts and Batches
We also have the CASE statement (aka SELECT CASE, DO CASE, and SWITCH/BREAK in other languages),
but it doesn’t have quite the level of control-of-flow capabilities that you’ve come to expect from other
languages.
The IF … ELSE Statement
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IF ... ELSE statements work much as they do in any language, although I equate them closest to C in
the way they are implemented. The basic syntax is:
IF
| BEGIN END
[ELSE
| BEGIN END]
The expression can be pretty much any expression that evaluates to a Boolean.
This brings us back to one of the most common traps that I see SQL programmers fall into — improper
user of NULLs. I can’t tell you how often I have debugged stored procedures only to find a statement like:
IF @myvar = NULL
Comp: above code is part of note.
This will, of course, never be true on most systems (see below), and will wind up bypassing all their
NULL values. Instead, it needs to read:
IF @myvar IS NULL
The exception to this is dependent on whether you have set the ANSI_NULLS option ON or OFF. The
default is that this is ON, in which case you’ll see the behavior just described. You can change this behavior
by setting ANSI_NULLS to OFF. I strongly recommend against this since it violates the ANSI standard.
(It’s also just plain wrong.)
Note that only the very next statement after the IF will be considered to be conditional (as per the IF).
You can include multiple statements as part of your control-of-flow block using BEGIN...END, but we’ll
discuss that one a little later in the chapter.
To show off a simple version of this, let’s run an example that’s very common to build scripts. Imagine
for a moment that we want to CREATE a table if it’s not there, but to leave it alone if it already exists. We
could make use of the EXISTS operator. (You may recall my complaint that the Books Online calls
EXISTS a keyword when I consider it an operator.)
-- We’ll run a SELECT looking for our table to start with to prove it’s not there
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’;