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Related to null: null space
See Null mutation.
Completely absent; a set with no members or of zero magnitude. If a field has a value of null, it means that the value is unknown. A null value is not the same as a value of zero, and the difference can be crucial. For example, when calculating the average value of a field among many records where one row contains a zero, the zero gets factored into the average; if the field has a null value, it does not get factored in to the average.