how to convert day data into week data

Question

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 

like

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