关于MySQL分组后显示数据的问题
有一个orderitems表如下:
+-----------+------------+---------+----------+------------+
| order_num | order_item | prod_id | quantity | item_price |
+-----------+------------+---------+----------+------------+
| 20005 | 1 | ANV01 | 10 | 5.99 |
| 20005 | 2 | ANV02 | 3 | 9.99 |
| 20005 | 3 | TNT2 | 5 | 10.00 |
| 20005 | 4 | FB | 1 | 10.00 |
| 20006 | 1 | JP2000 | 1 | 55.00 |
| 20007 | 1 | TNT2 | 100 | 10.00 |
| 20008 | 1 | FC | 50 | 2.50 |
| 20009 | 1 | FB | 1 | 10.00 |
| 20009 | 2 | OL1 | 1 | 8.99 |
| 20009 | 3 | SLING | 1 | 4.49 |
| 20009 | 4 | ANV03 | 1 | 14.99 |
+-----------+------------+---------+----------+------------+
然后,我输入查询:
select order_item, order_num from orderitems group by order_item;
显示的是:
+------------+-----------+
| order_item | order_num |
+------------+-----------+
| 1 | 20005 |
| 2 | 20005 |
| 3 | 20009 |
| 4 | 20009 |
+------------+-----------+
最后,
我的疑问:为什么没有输出所有的order_num,而只是输出了一部分