In SQL, NULL means "unknown value". Thus, every comparison with NULL yields NULL (unknown) rather than TRUE or FALSE.
From a conceptional point of view, this three-valued logic makes sense. From a practical point of view, every learner of SQL has, one time or another, made the classic
WHERE myField = NULL mistake or learned the hard way that
NOT IN does not do what one would expect when NULL values are present.
It is my impression (please correct me if I am wrong) that the cases where this three-valued logic helps (e.g.
WHERE myField IS NOT NULL AND myField <> 2 can be shortened to
WHERE myField <> 2) are rare and, in those cases, people tend to use the longer version anyway for clarity, just like you would add a comment when using a clever, non-obvious hack.
Is there some obvious advantage that I am missing? Or is there a general consensus among the development community that this has been a mistake?