Php WooCommerce SQL查询以显示客户日期之间的订单总数

这里有点纠结，为了得到一个SQL查询的正确输出，我正在编写一个SQL查询，以提取客户ID、总税金和订单总额。目前，我已经将每个订单单独取出，但当我按客户id分组时，它并没有像我预期的那样将所有总数相加

SELECT
    SUM( CASE WHEN pm.meta_key = '_order_total' AND p.ID = pm.post_id THEN pm.meta_value END ) AS order_total,
    SUM( CASE WHEN pm.meta_key = '_order_tax' AND p.ID = pm.post_id THEN pm.meta_value END ) AS order_tax,
    max( CASE WHEN pm.meta_key = '_customer_user' AND p.ID = pm.post_id THEN pm.meta_value END ) AS customer_id
FROM
    wp_posts p 
    join wp_postmeta pm ON p.ID = pm.post_id
    join wp_woocommerce_order_items oi ON p.ID = oi.order_id
WHERE
    post_type = 'shop_order' AND
    post_date BETWEEN '2015-01-01' AND '2016-07-08' AND
    post_status = 'wc-completed' 
GROUP BY
    p.ID 

我将p.ID替换为：

CASE WHEN pm.meta_key = '_customer_user' AND p.ID = pm.post_id THEN pm.meta_value END

但是我得到了以下结果（这是不正确的）

   Array
(
[0] => stdClass Object
    (
        [order_total] => 428984.019999999
        [order_tax] => 71247.3400000001
        [customer_id] => 
    )

[1] => stdClass Object
    (
        [order_total] => 
        [order_tax] => 
        [customer_id] => 15
    )

[2] => stdClass Object
    (
        [order_total] => 
        [order_tax] => 
        [customer_id] => 16
    )

[3] => stdClass Object
    (
        [order_total] => 
        [order_tax] => 
        [customer_id] => 19
    )

[4] => stdClass Object
    (
        [order_total] => 
        [order_tax] => 
        [customer_id] => 23
    )