assigning mysql value to variable inline -

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why doesn't work i'm trying previous , current value calculate percent change. both values correctly how can reuse them math operatio

when try below command error 1054 (42s22): unknown column 'currentval' in 'field list'

            select ifnull(dvalue,0) currentval,                        (select ifnull(dvalue,0)                         ...                        where...) previousval,                        (currentval-previousval)/previousval             ...             ...;

wrap query around have , calculate percentage there:

select currentval, previousval,         (currentval-previousval)/previousval percentchange     (select ifnull(dvalue,0) currentval,                    (select ifnull(dvalue,0)                         ...                        where...) previousval               ...               ...) t

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