Well, yes, this is how it's supposed to work, the values being defined as unknown. SQL is the same with NULLs not matching each other. Interestingly, this once

Well, yes, this is how it's supposed to work, the values being defined as unknown. SQL is the same with NULLs not matching each other. Interestingly, this once bit me the other way round. I get so used to NaN/NULL not matching itself that it's a bit off the radar to test, yet I discovered that SAS' equivalent concept, missing, does match with itself. Which caused some head scratching to find!