Joe Celko's SQL for Smarties, 3rd Edition

6.4. Comparing NULLs

A NULL cannot be compared to another NULL (equal, not equal, less than, greater than, and so forth). This is where we get SQL’s three-valued logic instead of two-valued logic. Most programmers do not easily think in three values. But think about it for a minute. Imagine that you are looking at brown paper bags and are asked to compare them without seeing inside of either of them. What can you say about the predicate “Bag A has more tuna fish than Bag B.”—is it TRUE or FALSE? You cannot say one way or the other, so you use a third logical value, UNKNOWN.

If I execute SELECT * FROM SomeTable WHERE SomeColumn = 2; and then execute SELECT * FROM SomeTable WHERE SomeColumn <> 2;, I expect to see all the rows of SomeTable between ...