题解 | #返回每个订单号各有多少行数#

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

1.count:count（列名）或count（*),区别就是第一个不计算该列null值

2.group by 聚合函数

3.order by 排序，默认升序