Question

I am having a bit of difficulty understanding how t-sql treats null values.

As a C# guy, I tend to want to do

IF(@myVar != null)...

But, this doesn’t ever appear to run my code. So I do

IF(@myVar is not null)

Whats the difference?

Second, the way addition works is unclear. Let’s say I have

declare @someCount int, @someFinalResult int

--Select that returns null
SELECT @someCount = columnName from tableName where someColumn = someValue

Then if I do

SET @someFinalResult = @someCount + 1--I seem to get NULL if I had null + something

But, if I first

declare @someCount int, @someFinalResult int
--FIRST SET DEFAULT TO 0
SET @someCount = 0

--Select that returns null
SELECT @someCount = columnName from tableName where someColumn = someValue

Now, @someCount defaults to 0, it’s not actually set equal to NULL even if the result is null. Why?

Answer 1

When you deal with NULL in SQL Server you basically work with 3-value logic with all the implications.

So in your example

IF(@myVar != null) vs IF(@myVar is not null)

It basically boils down to the question what is the difference between: @myVar = null vs @myVar is null

@myVar = null will always evaluate to null as what you are asking is:

is the value in @myVar equal to UNKNOWN

As you do not know what the UNKNOWN is this question cannot by answered yes, or no so it evaluates to UNKNOWN

e.g. 
    "is 1 = UNKNOWN" - I do not know
    "is 'a' = UNKNOWN" - I do not know 
    "is UNKNOWN = UNKNOWN" - I do not know

The last one may be a bit tricky but just imagine that you have 2 boxes with apples and you do not know neither how many apples are in box1 one nor in box2 so asking:

is count(box1) = count(box2)
is the same as 
is UNKNOWN = UNKNOWN"

so the answer is I do not know

the second one @myVar is null is different as it is like asking

is the value in @myVar UNKNOWN

so the difference is that you specifically ask “is it true that the value stored in the variable is UNKNOWN?”, so

    "is 1 UNKNOWN" - NO
    "is 'a' UNKNOWN" - NO
    "is UNKNOWN UNKNOWN" - YES