count函数+派生表和组-使其全部起作用

问题描述

有人可以帮我解释一下这个示例中SUM函数的去向吗？我试图获得返回列数的文字计数，而不是每个帖子ID的Meta_key数。我想要一个数字，在分组后有5个结果。

SELECT nmbr,post_id,DeliveryDate,DeliveryType
  FROM ( SELECT nmbr,MAX(CASE WHEN Meta_key = 'value_1' THEN Meta_value ELSE NULL END) as DeliveryDate,MAX(CASE WHEN Meta_key = 'value_2' THEN Meta_value ELSE NULL END) as DeliveryType
           FROM wp_postMeta
         GROUP 
             BY post_id 
       ) AS derived_table
       
 WHERE DeliveryDate >= CURRENT_DATE
   AND DeliveryType = 'delivery'

我已经尝试过了，但是它没有计数或求和，并且关于HAVING子句有些问题，导致没有结果返回。如果我删除它，它会在表上显示所有结果，但不计算它们。

SELECT
    post_id,COUNT(*) AS cnt,MAX(CASE WHEN Meta_key = 'value_1' THEN Meta_value END) AS DeliveryDate,MAX(CASE WHEN Meta_key = 'value_2' THEN Meta_value END) AS DeliveryType
FROM wp_postMeta
GROUP BY
    post_id
HAVING
    DeliveryDate >= CURRENT_DATE AND
    DeliveryType = 'delivery';

以下是上述内容产生的图像

解决方法

SELECT COUNT(*)
FROM (
SELECT nmbr,post_id,DeliveryDate,DeliveryType
  FROM ( SELECT nmbr,MAX(CASE WHEN meta_key = 'value_1' THEN meta_value ELSE NULL END) as DeliveryDate,MAX(CASE WHEN meta_key = 'value_2' THEN meta_value ELSE NULL END) as DeliveryType
           FROM wp_postmeta
         GROUP 
             BY post_id 
       ) AS derived_table
       
 WHERE DeliveryDate >= CURRENT_DATE
   AND DeliveryType = 'delivery'
   ) AS q