Alex-668179 (8/26/2010)

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AFAIK, NULL is the only way to leave a field empty!

I disagree: setting a field to NULL does not 'leave [or make] a field empty', instead it makes (or should make) any field value, and even the existence of such a value, undefined, inaccessible and irrelevant. That is what I meant before when I wrote before that NULL is a state: just stop thinking of NULL as a value!