--insert update分开吧，不然还要加些判断的
create trigger A_INS
on A
for insert,update
as
if update(F2) update t set t.B2=i.F2 from [DB2].[dbo].[B] t,inserted i where i.ID=t.ID;
insert into [DB2].[dbo].[B](B2,ID) select F2,ID from inserted A
    where not exists (select 1 from [DB2].[dbo].[B] t
        where t.ID=A.ID and t.F2=B.F2);
go