if not exists  (select 1 from part where partname ='C')
begin
    insert into part
    select max(id)+1 as ID,'C' from part
end
if not exists  (select 1 from part where partname ='D')
begin
    insert into part
    select max(id)+1 as ID,'D' from part
end
insert into relation
select  id ,producename,'C',1,'D',2
from produce where producename = 'H'

------解决方案--------------------
SQL code

if not exists  (select 1 from part where partname ='C')
begin
    insert into part
    select max(id)+1 as ID,'C' from part
end
if not exists  (select 1 from part where partname ='D')
begin
    insert into part
    select max(id)+1 as ID,'D' from part
end

--这一部分你插入成功了，下面你分开执行这一句
insert into relation
select  id ,producename,'C',1,'D',2
from produce where producename = 'H'

--这一部分执行有什么提示？