Filtering on a date range without selecting the date itself

We have a table of products, quantities, retail values
and sales dates . We want to select only the branches,
sum the quantities and sum the retail values by grouping them into a set of weekly date ranges to show sales in a
set of week columns
Problem is SQL asks for sale date to be in an aggregate
function or group by clause even though we don't select it
and sale dates vary so much we are getting duplicate entries on branches where we don't want them.
Any one done this ?
Tried to use an aggregate function on date but SQL still
says its not included etc ...
Our SQL expert is stuck on this too.