update from multiple rows

hi I have some trouble updating a table like this one : date     |     data_raw     |     data_sys 12-01   |   5
          |       4.5 13-01   |   6                   |       6 14-01   |   7                   |       8
 

I would like to update the 'data_sys' row by computing values of multiple 
'data_raw' values. I mean for example :data_sys(13-01) = (data_raw['12-01'] + data_raw['13-01'] + 
data_raw['14-01'] )/3;

I thought of a function that fetch the 3 data_raw rows for each rows.... 
but it was obviously too much slow...

Is there a more efficient way to achieve this ?
Thanks in advance.. This could help me very much..

Etienne Adam

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