How can I update this table by SQL?

How can I update this table (SCORE col):

name    date         DIV    SCORE    
------  -----------  -----  ---- 
TOM     2000-01-01   A        20   
TOM     2000-01-01   B        30   
TOM     2000-01-01   C       200
TOM     2000-01-03   A        10   
TOM     2000-01-03   B        40 
TOM     2000-01-03   C       300 
JACK    2000-01-01   C       500   
JACK    2000-01-01   B       100    
JACK    2000-01-01   A       200  
JACK    2000-12-01   C       300   
JACK    2000-12-01   B        50 
JACK    2000-12-01   A       100 

To:

name    date         DIV    SCORE    
------  -----------  -----  ---- 
TOM     2000-01-01   A        20   
TOM     2000-01-01   B        30   
TOM     2000-01-01   C       150(=200-20-30)
TOM     2000-01-03   A        10   
TOM     2000-01-03   B        40 
TOM     2000-01-03   C       250(=300-10-40) 
JACK    2000-01-01   C       200(=500-100-200)   
JACK    2000-01-01   B       100    
JACK    2000-01-01   A       200  
JACK    2000-12-01   C       150(=300-50-100)   
JACK    2000-12-01   B        50 
JACK    2000-12-01   A       100 

C-=A+B

Answer

In most databases you can use a correlated subquery:

update t
    set score = (score -
                 (select coalesce(sum(score), 0)
                  from t t2
                  where t2.name = t.name and t2.date = t.date and 
                        t2.div in ('A', 'B')
                 )
                )
    where div = 'C';

There are other ways to phrase this, but those are database-dependent.