TSQL Question here

Hello Gurus,

Please see attached  

There are two tables –
Metric_DTrange that specifies for a given combination of product and saleClass – the a code (AGMT) and gives its metric VolW. Then there is a blank column “metric_sum” within it as well

Daily_m – is a table that lists a given combination of product + salesClass + daily date ad lists out the metric for that date.

What is needed – I need tp update column “Metric_sum” of table metric_DTRange is such a way  - when I read for a given combination of product and salesClass in that table – I see the start and end dates. Then I go to the daily_m table to grab the [Metric] that lies within that date range  for the same combination of product + salesClass.
For example:
AGMT      Product      salesClass      st_date      end_date      VolW      Metric_sum
TRN01      P1      A      3/1/2014      3/10/2014      14-Feb      46
Where the metric sum comes to 46 – it sums all te metric between the dates of 3/1 and 3/10 within the daily_m table and puts it in the metric_DTrange
Book3.xlsx