or:
create trigger yourTriggerName on A
For insert
As
update B set 学分 = inserted.学分 from B
join inserted on b.学号 = inserted.学号
insert B select 学号,姓名,学分 from inserted
where 学号 not in (select 学号 from B)
create trigger yourTriggerName on A
For insert
As
if exists(select 学号 from B join inserted on b.学号 = inserted.学号)
update B set 学分 = inserted.学分 from B join inserted on b.学号 = inserted.学号
else
insert B select 学号,姓名,学分 from inserted