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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