missing or unknown data is considered as null values.It indicates that value is not applicable for a set of attributes. NULL Value is different from an empty or zero value
To test for NULL values we use comparison operators, such as =, <, or <>.
To select only the records with NULL values we use following syntax:
SELECT LastName,FirstName,city FROM emp
WHERE city IS NULL
LastName FirstName City
Tomar mukesh
Gupta ayush
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