Subject Re: [ib-support] how to perform update with aggregate subselect ?
Author Andrew Guts
My solution is:

delete from table1;
insert into table1 (CustomerID, ..., Total)
select t2.CustomerID, ..., sum(t2.Quantity) from table2 t2
where ...
group by t2.CustomerID, ...;

It works as expected.

Thanks all.

Andrew