The ISNULL function performs a comparison to determine if the result of the expression specified as an argument is NULL. The function returns 1 if it is NULL or 0 otherwise. You can check if a certain value is NULL. This function is working like the ISNULL expression.
ISNULL(expression)
--Using ISNULL function to select rows with NULL value
SELECT * FROM condition_tbl WHERE ISNULL(salary);
id name dept_name salary
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7 'Brown ' 'account' NULL