题意分析：

每个订单号（order_num）各有多少行数（order_lines），并按 order_lines对结果进行升序排序

  • 每个订单号：使用 group by 分组
  • 各有多少行数：count() 函数进行统计
select
    order_num,
    count(order_num) as order_lines
from
    OrderItems
group by
    order_num
order by
    order_lines