show difference between of two price list - php

Question

I have history list of product price(order by created_at) like this:
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
)
)
Now for show data in table I listed price using foreach method like this:
<?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;?>
I can see true output in table but I need to show difference between of two price in the third column like this picture:
how do can i show difference between of two price in my list?!

Answer 1

You just need to compare the current price property to the price from the previous object in the array?
Something like this should work:
<?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;?>

Answer 2

If you are running MySQL 8.0, you can compute this information directly in the database using window functions:
select
t.*,
price
- lag(price, 1, price) over(partition by product_id order by created_at)
as price_diff
from mytable t
This adds one more column to your resultset, that contains the difference between the current price and the previous price of the same product.