Why are empty fields omitted from != queries?

[Kimi Raikkonen]

I have a field called "fixVersion" I use to track when an issue was fixed or when it is scheduled to be fixed.  If I do a search for fixVersion != "6.0", the results show items where fixVersion is not assigned to "6.0", but omits items where fixVersion is empty.  Why does this query 'fixVersion != "6.0"' give me fewer results than this query 'fixVersion != "6.0" OR fixVersion is EMPTY'?  Shouldn't EMPTY also be not "6.0"?

[Norman Abramovitz]

In database terms, there is a difference between a field with a value and a field with no value(never set).   You can read why by looking up why SQL has an "IS NULL" condition.   You can see this effect on SQL when you try to do joins as well (ie. the difference between inner join and any one of the forms of outer joins)