how to convert day data into week data

i have a table in my database where i am storing data on the daily basis

i am displaying that data also in day base

but now i wanat to sum the days into the weeks to display less data

example

date product-name No.items-selled

01/01 Glaxy-x 10

01/02 Glaxy-x 5

01/02 Glaxy-y 30

01/03 Glaxy-x 1

01/04 Glaxy-x 14

01/05 Glaxy-x 25

01/06 Glaxy-x 5

01/07 Glaxy-x 10

now i want to display this

insted of date display week

date product-name No.items-selled

(week) 1 Glaxy-x 70

1 Glaxy-y 30

please help me and give me idea how i can retrive my data from database and convert it into week