select order_num,count(order_num) as order_lines
from OrderItems 
group by order_num 
order by order_lines

知识点：
1、count(*),count(列名)都可以，区别在于，count(列名)是统计非NULL的行数
2、order by最后执行，所以可以使用列别名
3、分组聚合一定不要忘记加上 group by ,不然只会有一行结果