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Null is not equal to zero.Even a value 0 is meaningful and can be used in computations. Null means an unknown value/something about which you dont have any idea/anything that is not applicable is treated as null. Say,in the EMP table if a certain employee is not entitled to earn commission,then for that row under that column we leave the value empty. Because no value is entered,there's no question of memory allocation. Its just like when you fill an application form ,you leave blank whichever fields are not applicable to you. They are called null values. You cannot compare a null value with another null value or another non-null value. Its not comparable with anything. Thats the reason why, we have to check for a null value using 'is null' condition;not using '=' operator. Hope,you find this answer agrreable to you.
Thanku
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