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What are the use of Null?

NULLs are basically for use in database for describe that no value or attributes are exist, 
But are not really values themselves. Instead they are just an indicator that a particular field has no value. Some discussions are clarifying that it is better than no value i mean blank value or a default value. we are individually can see little value in a blank field, but default values can be useful. The interesting point about NULLs is that no Primary Key can contain a NULL and thus it\'s useful when comparing Candidate Keys to see if one could potentially contain a NULL, thus disqualifying that key.

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