php - 项目名称相同的两个数值之间的差异

Question

我有一个名为“items”的表

id       month       year       **itemname**       distmoney
 1        12         2012         chicken           20
 2        12         2012         pork              15
 3        11         2012         chicken           21
 4        11         2012         pork              15

我试图找出两个月之间同一商品名称的“distmoney”之间的区别。

例子:

ID 1，元素名称鸡。第 12 个月的 distmoney 是 20，而第 11 个月的 distmoney 是 21。我希望能够计算 id=1，itemname=chicken 的 1 的差值

现在我有 php 代码来计算两个数字之间的差值，但我很难弄清楚如何获取上个月的 distmoney。

<?php foreach($rows as $row): ?>
<?php $number1 = htmlentities($row['distmoney']) ?>
<?php endforeach; ?>
<?php
$number1 = $row['distmoney'];
$number2 = ????????; // THIS NEEDS TO BE THE PREVIOUS MONTH DISTMONEY VALUE
if ($number1 <= $number2) {
$difference = "(Price Lowered) Price difference of $";
$result = $number2 - $number1;
$percent = round(100.0*($number2-$number1)/$number1);
echo $difference; echo $result; echo $percent; echo "%";
} elseif ($number1 > $number2) {
$result = $number1 - $number2;
$percent = round(100.0*($number2/$number1-1));
$addition = "(Price Higher) Price difference of $";
echo $addition; echo $result; echo $percent; echo "%";
} 

?>

Answer 1

如果您想在 SQL 中执行此操作，则可以转换列中的数据，然后获取差异:

select 
  itemname,
  max(case when month = 12 and year = 2012 then distmoney else 0 end) Dec2012,
  max(case when month = 11 and year = 2012 then distmoney else 0 end) Nov2012
from yourtable
group by itemname

参见 SQL Fiddle with Demo

查询结果为:

| ITEMNAME | DEC2012 | NOV2012 |
--------------------------------
|  chicken |      20 |      21 |
|     pork |      15 |      15 |