触发器约束表中数据重复

--表A中有两个字段CODE和CHANNELID 
--想写一个触发器，在添加或者修改的时候，不允许有CODE和CHANELID同时重复的
--比如 CHANNELID=1 的记录里面不能有两个CODE一样的，但是CHANNELID不同的话，CODE可以相等
--帮写个触发器或者约束，谢谢了
if object_id('表A') is not null
   drop table 表A
if object_id('tr_InsTableA') is not null
   drop trigger tr_InsTableA
if object_id('tr_UpdTableA') is not null
   drop trigger tr_UpdTableA
go
create table 表A(CODE varchar(10),CHANNELID int)
go
create trigger tr_InsTableA
on 表A
instead of insert
as
begin
   declare @CODE varchar(100),@CHANNELID int
   declare cur cursor for select CODE,CHANNELID from inserted
   open cur
   fetch next from cur into @CODE,@CHANNELID
   while(@@fetch_status=0)
     begin
        declare @i int
        select @i=count(1) from 表A where CODE=@CODE and CHANNELID=@CHANNELID
        print @i
        if(@i=1)
            begin
                 select * from 表A
            end
        else
            insert into 表A select @CODE,@CHANNELID
        fetch next from cur into @CODE,@CHANNELID
     end
    close cur
    deallocate cur
end

go
create trigger tr_UpdTableA
on 表A
instead of update
as
begin
   declare @CODE varchar(100),@CHANNELID int
   declare cur cursor for select CODE,CHANNELID from inserted
   open cur
   fetch next from cur into @CODE,@CHANNELID
   while(@@fetch_status=0)
     begin
        declare @i int
        select @i=count(1) from 表A where CODE=@CODE and CHANNELID=@CHANNELID
        print @i
        if(@i=1)
            begin
                 select * from 表A
            end
        else
            begin
                 declare @oldCODE varchar(100),@oldCHANNELID int
                 select @oldCODE=CODE,@oldCHANNELID=CHANNELID from deleted
                 update 表A set CODE=@CODE,CHANNELID=@CHANNELID
                      where CODE=@oldCODE and CHANNELID=@oldCHANNELID
            end
        fetch next from cur into @CODE,@CHANNELID
     end
    close cur
    deallocate cur
end