combo on condition

i have a combo list which contain 4 element "REF" "DF" "Oven" "WT" the combo is bind with sql table,
now i want if i select "REf" the system show REF modals in another combolist, i select DF show the DF modals.
for this purpose i have 2 tables in sql, one is bind with first combo and sec table have colomns ModalsID, Modal, and CategoryID,,
Kindly guide about the code who to do this
thanks in advance 
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