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comm!=300 in emp not working

When i tried 

select * from emp where comm=300;

giving 1 record.

In total emp table contains 14 records.

When i try 

select * from EMP where COMM !=300;

Getting only three records. Why i am not getting remaining records.

Please help anyone

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In emp table comm column we will have nulls. most of the functions (except nvl function) will not handle with null values.

So you must try below

select * from EMP where COMM!=300 or COMM is null;

or

select * from EMP where nvl(COMM,0)!=300 ;


You will get 13 records.
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