Update a running total field in table 1 from column in table 2

[sbr7770]

I have a table of Vendor invoices that has invoice number,po number and total invoice amount. I need to update another table that keeps a running total of the amount spent on the po from the vendor invoice table.
So I need to add field inv_tot_amount from table vn_inv to the running total field tot_1 in table ap_inv.

This is the statement I tried (along with many variations) and cannot seem to get it to work:

UPDATE ap_inv a SET tot_1= tot_1 + (select inv_tot_amount from vn_inv v (Select v.inv_num, v.po_num from vn_inv v where a.inv_num=v.inv_num AND a.po_num=v.po_num));
The message I get back is:
ERROR at line 1:
ORA-00907: missing right parenthesis

Is there a way to do this?

Thanks,
SBR

[tamilselvan]

UPDATE ap_inv a SET tot_1= tot_1 + (select inv_tot_amount from vn_inv v (Select v.inv_num, v.po_num from vn_inv v where a.inv_num=v.inv_num AND a.po_num=v.po_num));

UPDATE ap_inv a 
    SET tot_1= tot_1 + 
                    (select inv_tot_amount 
                       from vn_inv v,
                        (Select v.inv_num, v.po_num
                            from vn_inv v 
                           where a.inv_num=v.inv_num AND 
                            a.po_num=v.po_num
                        )
                     );

Comma missing.

Tamil