在MySQL中计算增值税/税金总额 - 增值税税率取决于地点字段

我正在尝试在MySQL中创建一个报告（来自Wordpress / Woocommerce安装中的表格，但这个商店征税的方式与Woocommerce的工作方式不同，所以我不能使用Woocommerce报告 . ）

计算需要从总数中计算出20％（英国增值税）的税率 - 但前提是客户位于特定地点 . 因此，如果客户处于“世界”状态，则税率为0％ . 如果客户在英国，税率为20％ . 但总收费仍然相同（因此出口的利润多于当地销售！）

我开始时：

SELECT TRIM(LEADING 'Order –' FROM post_title) AS Date,
m1.meta_value AS _order_number,
m2.meta_value AS _Location,
m3.meta_value AS _order_total
FROM wp_posts 
LEFT JOIN wp_postmeta AS m1 ON m1.post_id = wp_posts.ID AND m1.meta_key = '_order_number'
LEFT JOIN wp_postmeta AS m2 ON m2.post_id = wp_posts.ID AND m2.meta_key = '_shipping_method_title'
LEFT JOIN wp_postmeta AS m3 ON m3.post_id = wp_posts.ID AND m3.meta_key = '_order_total'
WHERE wp_posts.post_type = 'shop_order'

这给了我一张这样的表格：

Date     |  Order no  | Location    |  Order Total

May 2013 |  123       |World         |1124.00

Jan 2013 |  124       |UK            |163.00

到现在为止还挺好 . 那么我尝试添加一些SUM来计算增值税金额 .

SELECT TRIM(LEADING 'Order –' FROM post_title) AS Date,
m1.meta_value AS _order_number,
m2.meta_value AS _Location,
m3.meta_value AS _order_total,
SUM(m3.meta_value*20/120) AS _VAT,
SUM(m3.meta_value*100/120) AS _NET
FROM wp_posts 
LEFT JOIN wp_postmeta AS m1 ON m1.post_id = wp_posts.ID AND m1.meta_key = '_order_number'
LEFT JOIN wp_postmeta AS m2 ON m2.post_id = wp_posts.ID AND m2.meta_key = '_shipping_method_title'
LEFT JOIN wp_postmeta AS m3 ON m3.post_id = wp_posts.ID AND m3.meta_key = '_order_total'
WHERE wp_posts.post_type = 'shop_order'

我以为这会给我：

Date       |     Order no | Location  |    Order Total | VAT  |   NET

May 2013  |             123|   World |    1124.00  |    187.33 |   1311.33

Jan 2013  |         124  |    UK     |      163.00   |    27.17 |  190.17

然后我可以继续尝试找出如何按位置更改费率 . 不幸的是，似乎SUM一切，而不是逐行给我结果 .

1）我搞砸了什么把它总结在一起？

2）只有在_shipping_method_title = UK时才能最好地应用增值税计算的任何想法？

3）显然，这只适用于税率变化 . 处理这个问题是我最好的选择，只是在下次修改税率时按日期限制此报告，并为新税率适用的未来订单制作新报告？还是有一种更聪明的方式？

我很感激，向客户收取固定费率并支付可变税是一种有点奇怪的方法，但我不能改变这一点，我只是负责提供报告 . :-(

EDIT 感谢Gordon，我已经解决了我的SUM问题，现在我的查询看起来像这样并生成我期望的表格布局，如上所示：

SELECT TRIM(LEADING 'Order –' FROM post_title) AS Date,
m1.meta_value AS _order_number,
m2.meta_value AS _Location,
m3.meta_value AS _order_total,
SUM(m3.meta_value*20/120) AS _VAT,
SUM(m3.meta_value*100/120) AS _NET
FROM wp_posts 
LEFT JOIN wp_postmeta AS m1 ON m1.post_id = wp_posts.ID AND m1.meta_key = '_order_number'
LEFT JOIN wp_postmeta AS m2 ON m2.post_id = wp_posts.ID AND m2.meta_key = '_shipping_method_title'
LEFT JOIN wp_postmeta AS m3 ON m3.post_id = wp_posts.ID AND m3.meta_key = '_order_total'
WHERE wp_posts.post_type = 'shop_order'
GROUP BY wp_posts.ID

但我仍然不确定如何根据位置字段中的值更改乘数 . 这可能，或者我在这里咆哮错误的树？

EDIT 2

我已经找到了部分解决方案 - 而不是SUM行，我使用CASE给出了增值税的计算值 .

CASE 
        when m2.meta_value = 'World Delivery' then 0
        when m2.meta_value = 'UK Delivery' then  (m3.meta_value*20/120)
        when m2.meta_value = 'European Delivery' then  (m3.meta_value*20/120)
END AS _VAT

不幸的是，我不能只重复_NET值的CASE声明（价格 - 增值税，因为它与增值税的CASE相同，因此会产生错误 .

我想做点什么

SUM(_order_total-_VAT) AS _NET

但是* _order_total 和 _VAT *不会被识别为可以从中计算的列 .

FINAL EDIT 再次感谢Gordon，我的最终查询看起来像这样，我只是添加它以防其他人可能会发现它有用 . 此查询适用于Wordpress导出和报告wordpress插件，因此可以快速集成到Wordpress管理员并导出到Excel，这很漂亮 .

SELECT TRIM(LEADING 'Order –' FROM post_title) AS Date,
m1.meta_value AS _order_number,
m2.meta_value AS _Customer_Location,
m3.meta_value AS _order_total,

ROUND(SUM(case when m2.meta_value = 'World Delivery' then 0 else m3.meta_value*20/120 end),2) AS _VAT,
ROUND(SUM(case when m2.meta_value = 'World Delivery' then m3.meta_value else m3.meta_value*100/120 end),2) AS _NET
FROM wp_posts 
LEFT JOIN wp_postmeta AS m1 ON m1.post_id = wp_posts.ID AND m1.meta_key = '_order_number'
LEFT JOIN wp_postmeta AS m2 ON m2.post_id = wp_posts.ID AND m2.meta_key = '_shipping_method_title'
LEFT JOIN wp_postmeta AS m3 ON m3.post_id = wp_posts.ID AND m3.meta_key = '_order_total'
WHERE wp_posts.post_type = 'shop_order' AND wp_comments.comment_post_id = wp_posts.ID  AND wp_comments.comment_content="Order status changed from processing to completed."
GROUP BY wp_posts.ID