Another issue is that the concept of NULL is ill-defined, if you really think about it. There is usually no way to distinguish between use of NULL to represent

Another issue is that the concept of NULL is ill-defined, if you really think about it. There is usually no way to distinguish between use of NULL to represent (1) A missing value, (2) An unknown value, or (3) a default value. These are not trivial distinctions; knowing the semantic intent of a NULL can either hamper or aid in reasoning about a code-base and avoiding subtle errors