如何利用case when 做数据判空

SELECT 
    '%' valueUnit,
  CASE WHEN a.floating IS NULL   
  THEN '--' 
  WHEN a.floating =0
  THEN '--' 
  ELSE
   ROUND((b.floating-a.floating)/a.floating,2)*100
  END
   value,
   
  '标题' title
FROM
(
SELECT floating,updateTime FROM XXX ORDER BY updateTime DESC LIMIT 1
)b 

LEFT JOIN 
(
SELECT floating,updateTime FROM XXX ORDER BY updateTime DESC LIMIT 1,1
)a
ON 1=1

做计算的时候应该把字段做一个判断，不能为零。