It is invalid memeory location in computer memeory
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Very small.
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You mean SQL?
NULL = anything IS NULL
NULL <> anything IS NULL
...
NULL IS NULL = TRUE
NULL IS NOT NULL = FALSE
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An empty set is called a null set. Testing whether a set (or a
list, stack, queue) is empty or NULL is an essential concept in
both number theory and computer science.
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There is no null, it is just what it says when you log out.