COMO FAZER A PORCENTAGEM DE X BASEADO EM X + Y NO SQL

SELECT MONTH(workOrderDate) AS MES, 
       (SUM(CASE WHEN WorkType = '02' AND workOrderDescription = 'preventiva' THEN 1.0 ELSE 0 END)) AS preventiva,
       (SUM(CASE WHEN workOrderDescription = 'CORRETIVA' THEN 1 ELSE 0 END)) AS corretivas,
	   SUM(CASE WHEN WorkType = '02' AND workOrderDescription = 'preventiva' THEN 1.0 ELSE 0 END + CASE WHEN workOrderDescription = 'CORRETIVA' THEN 1 ELSE 0 END) AS total
	   FROM WorkOrder 
WHERE YEAR(workOrderDate) = 2018 AND lastUpdateData IS NOT NULL
GROUP BY MONTH(workOrderDate);