Question

我正在尝试构建一个SQL SELECT查询，该查询从两个不同的表中获取信息并按订单ID对它们进行分组。目前，我已经了解了这个问题：

SELECT T1.order_item_id, T2.order_item_id, T2.order_id, T3.post_id, 
       CASE WHEN T1.meta_key = 'Name' and T2.order_item_id = T1.order_item_id 
            THEN T3.meta_value 
            END as name, 
       CASE WHEN T3.meta_key = '_billing_address' and T2.order_id = T3.post_id 
            THEN T3.meta_value 
            END as billing_address, 
       CASE WHEN T3.meta_key = '_billing_email' and T2.order_id = T3.post_id 
            THEN T3.meta_value 
            END as billing_email 
FROM wp_woocommerce_order_itemmeta T1 
LEFT JOIN wp_woocommerce_order_items T2 
       ON T1.order_item_id = T2.order_item_id 
LEFT JOIN wp_postmeta T3 
       ON T2.order_id = T3.post_id

以下是我从中获取的表格：

T1：

 +---------------------------------------+
 | order_item_id | meta_key | meta_value|
 +---------------------------------------+
 | 1             | Name     | John Doe   |                      
 +---------------------------------------+

T2

 +---------------------------------------+
 | order_item_id  |  order_id            |
 +---------------------------------------+
 | 1              |  101                 |
 +---------------------------------------+

T3

 +-------------------------------------------------+
 | post_id   | meta_key         | meta_value       |
 +-------------------------------------------------+
 | 101       | _billing_address | 123 Fake St.     |
 | 101       | _billing_email   | jondoe@gmail.com |
 +-------------------------------------------------+

我希望能够创建一个如下所示的表：

 +-------------------------------------------------------+
 | order_id | name     | address      | email            |
 +-------------------------------------------------------+
 | 101      | John Doe | 123 Fake St. | jondoe@gmail.com |                                 
 +-------------------------------------------------------+

但是上述SQL查询产生的表有很多用于帐单地址，名称和电子邮件的空值，而且似乎并没有从单元格中获取数据。

我觉得CASE表达可能是个问题，但我并不完全确定。

Answer 1

你有条件部分，现在使用UNUserNotificationCenter.current().getPendingNotificationRequests(completionHandler: { requests in for request in requests { print(request.trigger) } })添加一些聚合来通过条件聚合来解决这个问题：

max()

看起来您也可以像这样简化案例陈述：

SELECT 
    T1.order_item_id
  , T2.order_item_id
  , T2.order_id
  , T3.post_id
  , max(CASE WHEN T1.meta_key = 'Name' and T2.order_item_id = T1.order_item_id 
            THEN T3.meta_value 
            END) as name
  , max(CASE WHEN T3.meta_key = '_billing_address' and T2.order_id = T3.post_id 
            THEN T3.meta_value 
            END) as billing_address
  , max(CASE WHEN T3.meta_key = '_billing_email' and T2.order_id = T3.post_id 
            THEN T3.meta_value 
            END) as billing_email 
FROM wp_woocommerce_order_itemmeta T1 
  LEFT JOIN wp_woocommerce_order_items T2 
    ON T1.order_item_id = T2.order_item_id 
  LEFT JOIN wp_postmeta T3 
    ON T2.order_id = T3.post_id
group by 
    T1.order_item_id
  , T2.order_item_id
  , T2.order_id
  , T3.post_id

Answer 2

我认为如果您在选择的postmeta表上两次加入联接，则查询会更容易：

SELECT T1.order_item_id, T2.order_item_id, T2.order_id, T3.post_id, 
T1.meta_key as name, 
T3A.meta_key as address, 
T3E.meta_key as email, 

FROM wp_woocommerce_order_itemmeta T1 
LEFT JOIN wp_woocommerce_order_items T2 
       ON T1.order_item_id = T2.order_item_id 
LEFT JOIN wp_postmeta T3A 
       ON T2.order_id = T3A.post_id AND T3A.meta_key = '_billing_address'
LEFT JOIN wp_postmeta T3E
       ON T2.order_id = T3E.post_id AND T3E.meta_key = '_billing_email'

注意：此查询未经过测试，但您可以提供示例数据库

SQL LEFT JOIN与CASE表达式

2 个答案: