if exists(
    select * from table where 学号=xxx)
   update......
else
   insert......

------解决方案--------------------
update a set col = b.col from a, b where a.id = b.id

insert into a select * from b where id not in (select id from a)

------解决方案--------------------
if exists(select * from table where 学号=xxx )
begin
  update......
end
else
begin
  insert......
end

------解决方案--------------------
insert into ....on duplicate key update...