Recently, I’ve decided to try to catch up on some things that were “snuck” into SQL Server while I wasn’t paying attention. The first post in this series was about TRY_CONVERT() (which is awesome) and today I want to talk about IIF().

I can’t imagine that there is a single beginner who has written any appreciable amount of SQL queries who at some point didn’t have to Google “how to do an if statement in sql” or ask someone with a little more experience. I certainly know that I had to do that. What you end up with is the CASE statement.

Let’s create a table to work with and populate it with some data.

USE tempdb
GO

IF OBJECT_ID('Customer') IS NULL
BEGIN
	CREATE TABLE dbo.Customer (
		CustomerId INT IDENTITY(1,1) PRIMARY KEY CLUSTERED,
		Name NVARCHAR(100) NOT NULL,
		CreditLimit MONEY NOT NULL,
		Active BIT NOT NULL		
	);

	INSERT dbo.Customer (Name, CreditLimit, Active)
	SELECT 'Nike', 10000000.00, 1
	UNION
	SELECT 'Under Armor', 5000000.00, 1
	UNION
	SELECT 'Reebok', 1000000.00, 1
	UNION
	SELECT 'Buster Brown', 10000.00, 0
END
GO

If we select out all of the data from the table now, it should look like this:

So, if we were to run a standard CASE statement against the table, it might look something like this. This statement returns the name of all of the customers along with their “Credit Status”. If a credit limit is greater than 1,000,000, then they are Preferred. Otherwise, they are Standard.

SELECT Name, CASE WHEN CreditLimit > 1000000.00 THEN 'Preferred' ELSE 'Standard' END AS CreditStatus
FROM dbo.Customer

This returns the following results.

T-SQL finally has an if statement of sorts for use in SELECT statements (logical branching IFs have always existed… I used one in my table create script). Those of you with any kind of VBA background (Access, Excel Macros and Formulas, etc) are probably already familiar with IIF(), which stands for “Immediate IF”. The T-SQL version takes three parameters, just as the VBA version does. The first parameter is the conditional, the second is what will display if the conditional is true, and the third is what will display if the conditional is false. Here is that same Credit Status query rewritten to use IIF().

SELECT Name, IIF(CreditLimit > 1000000.00, 'Preferred', 'Standard') AS CreditStatus
FROM dbo.Customer

You can see that the results are the same and the query does look a little cleaner.

What if you want to do multiple “layers” of the conditional? Nested IIF statements! Take a look at this query:

SELECT Name, 
IIF(CreditLimit > 5000000.00, 'Platinum', 
	IIF(CreditLimit >= 1000000.00, 'Gold', 'Silver')) AS CreditStatus
FROM dbo.Customer

The equivalent CASE statement would look like this:

SELECT Name, 
CASE 
	WHEN CreditLimit > 5000000.00 THEN 'Platinum'
	WHEN CreditLimit >= 1000000.00 THEN 'Gold'
	ELSE 'Silver'
END AS CreditStatus
FROM dbo.Customer

Both of those return a result set like this:

So, those statements do the same thing. For complicated conditional logic (especially logic that requires 5 or more decisions), I’d much prefer the CASE statement. Nested IIFs gets pretty dirty once you get that far. However, that appears to be an entirely stylistic choice. If you have any strong preference one way or the other, comment and let me know how you feel and why.

I am a little behind the curve on this one. Starting in SQL Server 2012, Microsoft introduced the TRY_CONVERT() function. This function works the same way as the CONVERT() function, but if the convert doesn’t work, you get NULL instead of an error.

Let’s take a look at the old way:

SELECT CONVERT(INT, 'aaa')

This returns the message “Conversion failed when converting the varchar value ‘aaa’ to data type int.” If you are doing a CONVERT() on some column in a large dataset, this often can throw you while you try to sort out what went wrong. Enter TRY_CONVERT(). Here is the new syntax:

SELECT TRY_CONVERT(INT, 'aaa')

This just returns NULL and doesn’t error at all. In this case, you can easily provide a default value for the conversion by using ISNULL() or COALESCE()

SELECT ISNULL(TRY_CONVERT(INT, 'aaa'), 0)
-- OR
SELECT COALESCE(TRY_CONVERT(INT, 'aaa'), 0)

Pretty darn awesome and about time (well, even four years ago, it was about time!).