I have two tables:

Table 1:
Item Sales
I_1 5
I_2 7
I_1 3
I_3 6

Table 2:
Item Group
I_1 G_1
I_2 G_2
I_3 G_1

I have no problem to calculate sales of certain item
=SUMPRODUCT(--(A1:A5="I_1");B1:B5). But I dont know how to calculate sales of
items from certain group. I dont want to manipulate table 1 because I get one
table like this every day. Any help appreciated.