在求一条插入话语···_Sql Server

在求一条插入语句···
有三张表
产品表（produce）主键是id
id producename
1 E
零件表（part）主键是id
id partname
1 A
2 B

产品关系表（relation）外键 produceid 对应的是 produce表的id
produceid producename partname1 number1 partname2 number2
1 E A 1 B 2

这是说一个产品E需要1个A和两个B才能生产

现在需要添加产品H 一个H需要一个零件C和两个零件D才能生产··

插入零件C的时候还需要判断零件表中是不是存在C··如果存在就不插入，如果不存在，就把数据插入part表

怎么用一条SQL语句就写出来···求大神解答··

------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 partendif not exists  (select 1 from part where partname ='D')begin    insert into part    select max(id)+1 as ID,'D' from partendinsert into relationselect  id ,producename,'C',1,'D',2from produce where producename = 'H'
------解决方案--------------------
SQL codeif not exists  (select 1 from part where partname ='C')begin    insert into part    select max(id)+1 as ID,'C' from partendif not exists  (select 1 from part where partname ='D')begin    insert into part    select max(id)+1 as ID,'D' from partend--这一部分你插入成功了，下面你分开执行这一句insert into relationselect  id ,producename,'C',1,'D',2from produce where producename = 'H'--这一部分执行有什么提示？