题解 | #返回每个订单号各有多少行数#
返回每个订单号各有多少行数
https://www.nowcoder.com/practice/cf1f8d4a514d455aa0468718fb411f41
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
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