Return Null If A Value Is A Certain Value

You need to return NULL only if the value of your data is a certain value. How do you do this?
There are three different ways.

NULLIF
DECLARE @1 char(1)
SELECT @1 ='D'


SELECT NULLIF(@1,'D')


REPLACE
This should not really be used, I just added it here to demonstrate that you can in fact use it.

DECLARE @1 char(1)
SELECT @1 ='D'

SELECT REPLACE(@1,'D',NULL)


CASE
With case you can test for a range of values. You can test for example for values between A and D. If you reverse the logic then you also don't need to provide the ELSE part since it defaults to NULL anyway.

DECLARE @1 char(1)
SELECT @1 ='D'


SELECT CASE @1 WHEN 'D' THEN NULL ELSE @1 END

--No else needed
SELECT CASE WHEN @1 <> 'D' THEN @1 END

And this is how you test for a range.

--Null
DECLARE @1 char(1)
SELECT @1 ='D'

SELECT CASE WHEN @1 BETWEEN 'A' AND 'D' THEN NULL ELSE @1 END

--E
DECLARE @1 char(1)
SELECT @1 ='E'

SELECT CASE WHEN @1 BETWEEN 'A' AND 'D' THEN NULL ELSE @1 END

Do you know how NULLIF and non-deterministic functions work?

Run this first

CREATE TABLE #j (n varchar(15))

DECLARE @a int
SET @a = 1
WHILE @a <= 1000 BEGIN
INSERT #j
SELECT NULLIF(REPLICATE('1', RAND()*2) , ' ')
SET @a = @a + 1
END
Go

After that is done run this query

SELECT * FROM #j WHERE n = ' '

You will get back between 200 and 300 rows. What just happened? In our insert we use this NULLIF(REPLICATE('1', RAND()*2) , ' ')
What this does is the following: if REPLICATE('1', RAND()*2) equals ' ' then it will insert a NULL, so where do the blanks come from? Well let's find out.

Run this

SET SHOWPLAN_TEXT ON

SELECT NULLIF(REPLICATE('1', RAND()*2) , ' ')

and we see the following
--Compute Scalar(DEFINE:([Expr1000]=If (replicate('1', Convert(rand(NULL)*2))=' ') then NULL else replicate('1', Convert(rand(NULL)*2))))

This can also be written like this

SELECT CASE WHEN REPLICATE('1', RAND()*2) =' '
THEN NULL ELSE REPLICATE('1', RAND()*2) END

See what happens? First SQL evaluates if REPLICATE('1', RAND()*2) is ' ' if that is not ' ' then the same code gets executed again and thus could be a blank
If you use a variable this can never happen.

Here is an example, keep hitting F5 and you will see it will never be blank

DECLARE @val float
SET @val = RAND()

SELECT NULLIF(REPLICATE('1', CONVERT(int, @val*2)) , ' ')