根据MySQL中的上一行更新值

Question

这个问题是基于下面我的上一个问题。 Get percentage based on value in previous row

与其获取一条select语句， 我希望更新语句使用更新查询来填充朗姆酒列。

输入：

+---+--------+------+
|CID| number | rum  |
+---+--------+------+
| 1 | 1.0000 | NULL |
| 3 | 2.0000 | NULL |
| 5 | 2.0000 | NULL |
| 6 | 4.0000 | NULL |
+---+--------+------+

输出：

+---+--------+------+
|CID| number | rum  |
+---+--------+------+
| 1 | 1.0000 | NULL |
| 3 | 2.0000 |100.0 |
| 5 | 2.0000 |  0.0 |
| 6 | 4.0000 |100.0 |
+---+--------+------+

公式：

rum = (currNumber - prevNumber) / prevNumber * 100

Answer 1

您可以在JOIN中使用UPDATE和窗口函数。一种方法是：

update t join
       (select t.*, lag(number) over (order by cid) as prev_number
        from t
       ) tt
       on tt.cid = t.cid
    set rum = (number - prev_number) * 100 / prev_number;