SQL Server select variable where no results - sql

Question

Can I set variables if the query returns no results?
DECLARE #Output AS VARCHAR(MAX);
SELECT #Output = 'Old';
SELECT #Output = 'New' WHERE 0 = 1;
PRINT #Output;
Output value is: Old
Expected value is: NULL

Answer 1

DECLARE #Output AS VARCHAR(MAX);
SELECT #Output = 'Old';
SELECT #Output = 'New' WHERE 0 = 1;
PRINT #Output;
You get 'Old' cause the variable already has this value, and it will not update the value since in the WHERE clause you use the condition 0=1, which will return FALSE and the value in the variable won't change.
WHERE 0 = 1 It will be False
WHERE 0 <> 1 It will be True
It's just similar to IF 0=1 THEN UpdateMyVar
So in your case the value will always 'Old', it won't return 'New' or NULL either.
I don't know what are you trying to do really, but if you want to return NULL then
DECLARE #Output AS VARCHAR(MAX);
SELECT #Output = 'Old';
SELECT #Output = NULL WHERE 0 <> 1; --If 0<> 1 then update my var and set NULL else leave it as it is
PRINT #Output;

Answer 2

Try this:
DECLARE #Output AS VARCHAR(MAX);
SELECT #Output = 'Old';
SELECT #Output = CASE 0 WHEN 1 THEN 'New' ELSE NULL END
SELECT #Output;

Answer 3

Sami provides a good explanation of what is going on.
If you want to be sure that a value is assigned, then you need to be sure that the query returns one row for the assignment. One way to do this is to use aggregations:
DECLARE #Output AS VARCHAR(MAX);
SELECT #Output = 'Old';
SELECT #Output = MAX('New') WHERE 0 = 1;
SELECT #Output;

Answer 4

This is simply how assigning variables in SELECT is implemented in SQL Server. As #SMor said in the comments, use SET instead of SELECT.
DECLARE #Output AS VARCHAR(MAX);
SET #Output = 'Old';
SET #Output = (SELECT 'New' WHERE 0 = 1);
-- this would change the #Output value to NULL
SELECT #Output;
The only problem with SET is that it can assign only one variable, while in SELECT you can assign many variables at the same time.