Total over different fields

[#1]

Hi, I want to write the following expression:

sum(aggr(if( INV_HIST_STORE_ON_HAND > 100,

sum(total<HIERARCHY, WEEK> TRX_NET_UNITS)

/

sum(total<HIERARCHY> TRX_NET_UNITS)

),ITEM_SKU, WEEK))

where HIERARCHY is a field that for each ITEM_SKU contains the name of one of 4 fields :

SUB_CLASS_NAME

CLASS_NAME

SUB_DEPARTMENT_NAME

DEPARTMENT_NAME

So that the expression would total over the field contained in HIERARCHY for each ITEM_SKU.

Is this possible, or is there an alternative that does not use nested if statements or pick?