if not exists(select 1 from table1 where id = @id and (a=1 and @a=3 or a=3 and @a = 1))
begin 
  insert into table1 (a,id) values(@a,@id);
  update table2 set goods = goods + 1 where a = @a;
end
else
begin
  delete from table1 where id = @id;
  update table2 set goods = goods - 1 where a = @a;
end