the difference between Null and not Null

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MySQL

Hi there,

What is the main difference between Null and Not Null.

Thanks,
Enos

Posted 11-Jul-12 4:20am

Nkhanedzeni

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OriginalGriff · Accepted Answer · 2012-07-11T04:27:00

Null is an absence of a value, so Not Null is the absence of the absence of a value.

I.e. if there is no value, it it Null.
If there is any value, it is Not Null.

Null should not be confused with zero, or naught, as that is a value. Null is a special condition which is outside the range of values of any item. For example, if you remove 1, 2 and 3 from the set {1, 2, 3} you get the empty or null set {}. If you remove 1, 2 and 3 from the set {0, 1, 2, 3} you do not - you get a set with a single element in it: {0}

[edit]Typos: "teh" for "the", "absnce" for "absence" - OriginalGriff[/edit]