IF IS NOT / <> NULL

IF i_earnings_amount IS NOT NULL THEN

        l_earnings_amount := TO_CHAR(i_earnings_amount, '9999.99');

END IF;

IF i_earnings_amount <> NULL THEN

        l_earnings_amount := TO_CHAR(i_earnings_amount, '9999.99');

END IF;

There is a significant difference between the two statements. IS NOT NULL, tests whether the field/variable has a value. (TODO: further clarify this issue)