FWIW, this is how I'd probably do it. I like to get rid of NULLs as early as possible in a procedure so I don't have to use "Where X IS [NOT] NULL" syntax. Sometimes NULLs are unavoidable and of course this snippet assumes an INT key > 0.

DECLARE @id INT

SET @id = ISNULL(@id,0)

IF @id < 0 SET @id = 0

UPDATE myTable

SET ClearFlag = 1

WHERE

(ID = @id)

OR

(ClearFlag = 0 AND @id = 0)