if 条件 then
    -- SQL语句;
else
    -- SQL语句;
end if;

delimiter $$
-- 创建语句    
create trigger my_tri after insert on orders for each row
begin
    -- 获取表中的商品库存inv到变量@inv中
    select inv from goods where id = new.g_id into @inv;
    -- 比较库存是否充足
    if @inv < new.g_number then
        -- 库存不足，暴力报错
        insert into xxx values(xxx);
    end if;
    update goods set inv = inv - new.g_number where id = new.g_id;
end
$$
delimiter ;