Question

我的下表中 parent_id ，价格，数量和 exp_date 是主要字段

+-----------+-------+----------+------------+---------------------+------------------+
| parent_id | price | quantity | exp_date   | last_modified       | last_modified_by |
+-----------+-------+----------+------------+---------------------+------------------+
| 2         | 410   | 3.00     | 2016-07-30 | 2016-07-22 18:14:34 | test             |
+-----------+-------+----------+------------+---------------------+------------------+
| 2         | 300   | 10.00    | 0000-00-00 | 2016-07-22 18:14:34 | test             |
+-----------+-------+----------+------------+---------------------+------------------+
| 2         | 540   | 17.00    | 2016-07-22 | 2016-07-22 18:14:34 | test             |
+-----------+-------+----------+------------+---------------------+------------------+

我想从上一个修改行开始，从总数量中减去一个不同的值。现在我有这个问题：

SET @remain = -19;
Update Stock_props SET quantity =
(SELECT IF((@remain := quantity+@remain) < 0,'0',@remain) as quantity)
WHERE parent_id = 2
ORDER BY last_modified DESC

这个特殊的工作正在起作用，因为我减去的值大于最后一行。它将输出：

+-----------+-------+----------+------------+---------------------+------------------+
| parent_id | price | quantity | exp_date   | last_modified       | last_modified_by |
+-----------+-------+----------+------------+---------------------+------------------+
| 2         | 410   | 0.00     | 2016-07-30 | 2016-07-22 18:14:34 | test             |
+-----------+-------+----------+------------+---------------------+------------------+
| 2         | 300   | 0.00     | 0000-00-00 | 2016-07-22 18:14:34 | test             |
+-----------+-------+----------+------------+---------------------+------------------+
| 2         | 540   | 11.00    | 2016-07-22 | 2016-07-22 18:14:34 | test             |
+-----------+-------+----------+------------+---------------------+------------------+

但是如果我想减去像11这样的较小量，例如结果将是这样的：

+-----------+-------+----------+------------+---------------------+------------------+
| parent_id | price | quantity | exp_date   | last_modified       | last_modified_by |
+-----------+-------+----------+------------+---------------------+------------------+
| 2         | 410   | 2.00     | 2016-07-30 | 2016-07-22 18:14:34 | test             |
+-----------+-------+----------+------------+---------------------+------------------+
| 2         | 300   | 0.00     | 0000-00-00 | 2016-07-22 18:14:34 | test             |
+-----------+-------+----------+------------+---------------------+------------------+
| 2         | 540   | 19.00    | 2016-07-22 | 2016-07-22 18:14:34 | test             |
+-----------+-------+----------+------------+---------------------+------------------+

而不是：

+-----------+-------+----------+------------+---------------------+------------------+
| parent_id | price | quantity | exp_date   | last_modified       | last_modified_by |
+-----------+-------+----------+------------+---------------------+------------------+
| 2         | 410   | 0.00     | 2016-07-30 | 2016-07-22 18:14:34 | test             |
+-----------+-------+----------+------------+---------------------+------------------+
| 2         | 300   | 2.00     | 0000-00-00 | 2016-07-22 18:14:34 | test             |
+-----------+-------+----------+------------+---------------------+------------------+
| 2         | 540   | 17.00    | 2016-07-22 | 2016-07-22 18:14:34 | test             |
+-----------+-------+----------+------------+---------------------+------------------+

我在这里缺少什么？提前谢谢！

Answer 1

好吧，所以如果有人在将来偶然发现这个问题，那么下面的查询就会完美地运作：

ALTER TABLE Stock_props ADD helper numeric;
SET @remain = 11;
Update Stock_props SET quantity =
(SELECT IF(((@remain := quantity+@remain) < 0),0,@remain) as quantity),
helper = (SELECT IF((@remain>0), @remain:=0,@remain)as helper),
ORDER BY last_modified DESC;
ALTER TABLE Stock_props DROP helper;

以上内容如下：

在名为helper
设置数量和@remain的值（后者是必要的，因为如果它是负数我们想继续减法）
根据符号
删除辅助行

MySQL从一个查询中的多行中减去

1 个答案: