Group By More Than One Field

Hi!

I would like to know how GroupBy works. I have a 1-many relationship (invoice-products)

I have this query:

[OPERATION_ID], DESCRIPTION: [PRODUCT NAME] & [PRODUCT TYPE], [QUANTITY]

What I would like is to have a group by DESCRIPTION since some products may be repeated in a same OPERATION cuz they are from dif origin but in the invoice they gotta be merged.

So if I have:

1 | GARLIC WHITE | 20
1 | GARLIC WHITE | 20
1 | ONION RED | 20
2 | GARLIC WHITE | 20
2 | POTATOE RED | 20

The query should return

1 | GARLIC WHITE | 40
1 | ONION RED | 20
2 | GARLIC WHITE | 20
2 | POTATOE RED | 20

With this I mean: If DESCRIPTION (product and product type) are the same, group them Per OPERATION_ID and Sum QUANTITIES