显示两个价目表之间的差异

Question

我有这样的产品价格历史列表（按 created_at 排序）：

Array
(
    [0] => stdClass Object
        (
            [id] => 1
            [product_id] => 49
            [price] => 14520000.0000
            [created_at] => 1592501154
        )

    [1] => stdClass Object
        (
            [id] => 2
            [product_id] => 49
            [price] => 14000000.0000
            [created_at] => 1592587554
        )

    [2] => stdClass Object
        (
            [id] => 4
            [product_id] => 49
            [price] => 14800000.0000
            [created_at] => 1592673954
        )

    [3] => stdClass Object
        (
            [id] => 5
            [product_id] => 49
            [price] => 10000000.0000
            [created_at] => 1592760354
        )

    [4] => stdClass Object
        (
            [id] => 6
            [product_id] => 49
            [price] => 14000000.0000
            [created_at] => 1592846754
        )

    [5] => stdClass Object
        (
            [id] => 7
            [product_id] => 49
            [price] => 14000000.0000
            [created_at] => 1592933154
        )

    [6] => stdClass Object
        (
            [id] => 8
            [product_id] => 49
            [price] => 14000000.0000
            [created_at] => 1593019554
        )

)

现在为了在表格中显示数据，我使用foreach 方法列出价格，如下所示：

   <?php foreach($product_prices_list as $product_price_list):?>
     <tr>
         <td><?= esc($product_price_list->created_at);?></td>
         <td class="text-center"><?= esc(number_format($product_price_list->price));?></td>
         <td class="text-center"></td> //show difference between of two price
     </tr>
   <?php endforeach;?>

我可以在表格中看到真实的输出，但我需要在第三列中显示两个价格之间的差异，如下图所示：

如何在我的列表中显示两个价格之间的差异？！

Answer 1

如果您运行的是 MySQL 8.0，您可以使用窗口函数直接在数据库中计算此信息：

select
    t.*,
    price 
        - lag(price, 1, price) over(partition by product_id order by created_at) 
        as price_diff
from mytable t

这会在您的结果集中再增加一列，其中包含同一产品的当前价格与之前价格之间的差异。

Answer 2

您只需要将当前价格属性与数组中前一个对象的价格进行比较？

这样的事情应该可以工作：

<?php foreach($product_prices_list as $key => $product_price_list):?>
    <tr>
        <td><?= esc($product_price_list->created_at);?></td>
        <td class="text-center"><?= esc(number_format($product_price_list->price));?></td>
        <td class="text-center"><?= (!empty($product_prices_list[$key - 1])) ? $product_prices_list[$key + 1]->price - $product_price_list->price: 0; ?></td> //show difference between of two price
    </tr>
<?php endforeach;?>