du kan inte referera till en aliasad kolumn i samma SELECT, du måste lägga den i en underfråga:
SELECT currentVal, previousVal, (currentVal-previousVal)/previousVal
FROM (
SELECT IFNULL(DValue,0) as currentVal,
(SELECT IFNULL(DValue,0)
FROM ...
WHERE...) as previousVal,
FROM ...
WHERE ...) T;