if not exists(select 1 from a where 名字='c')
insert into a select 名字,年龄,...
insert into b select a_id,.成绩 
else
update a set ...
insert into b select .. where not exists(select 1 from b where 成绩=@成绩 and 学科=@学科)

------解决方案--------------------

SQL code

declare @CId int
set @CId=0
select @CId=ID from A where [名字]='C'
if (@CId>0)
begin
    update A set ... where [名字]='C'
    if not exists (select top (1) * from B where A_id=@CId and [学科]='语文')
    begin
        insert into B (A_id,[学科],[成绩]) values (@CId,'语文',85)
    end
end
else
begin
    insert into A ([名字],[年龄]) values ('C',21)
    set @CId =SCOPE_IDENTITY()
    insert into B (A_id,[学科],[成绩]) values (@CId,'语文',85)
end