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In databases NULL means the value is unknown and therefore NULL == NULL is false since two unknowns are rarely going to be the same value if you ever find out what they are. So testing for = NULL will never find a match as NULL is never equal to anything.
Once you move out of the database null has a completely different meaning and two fields set to null are equal. The NULL in the database doesn't extract as a meaningful value though because outside the database there is no corresponding value meaning unknown to give the field.
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