Calculate the ratio of disabled students who have never been absent from school.
ratio = MULTIPLY(DIVIDE(SUM(month = 0), COUNT(disabled.name)), 100); never been absent from school refers to month = 0;
SELECT 100 * SUM(IIF(T2.month = 0, 1, 0)) AS num FROM disabled AS T1 INNER JOIN longest_absense_from_school AS T2 ON T1.name = T2.name