I am trying to compare a column col1 and a variable @myvar in a WHERE clause. Both usually contain GUIDs, but may also have NULL values.
I thought I could get around the fact that NULL=NULL evaluates to FALSE by using WHERE ISNULL(col1, '')=ISNULL(@myvar, ''). That would compare two empty strings instead, and evaluate to TRUE.
This will, however, produce the following error message:
Msg 8169, Level 16, State 2, Line 3 Conversion failed when converting from a character string to uniqueidentifier.
I tried
DECLARE @myvar uniqueidentifier = NULL
SELECT ISNULL(@myvar,'') as col1
Same error message.
Two questions: First, I am trying to convert a uniqueidentifier variable - even though it has a NULL value - to an (empty!) string, not the other way around, as the error message suggests. What gives?
Second, is there a better way to word that WHERE clause I need, to allow for comparing uniqueidentifiers that might be NULL?
Since the first argument you are passing isnull is not a literal null, it will determine the return type of that call, a uniqueidentifier in your case. The second argument, '', cannot be cast to this type, hence the error you're getting.
One way around this is just to explicitly check for nulls:
WHERE (@myvar IS NULL AND col1 IS NULL) OR (col1 = @myvar)