count() 函数 + derived table 和 Group - 使一切正常

Question

有人可以帮我解释一下这个例子中 SUM 函数的位置吗？我正在尝试对 returned 列的数量进行字面计数，而不是每个 post id 的 meta_key 的数量。我想要一个数字，就像分组后有 5 个结果一样。

SELECT nmbr, post_id
     , DeliveryDate
     , DeliveryType
  FROM ( SELECT nmbr, post_id
              , 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，但它没有计算或求和任何东西，HAVING 子句中有些东西导致 return 没有结果。如果我删除它，它会在 table 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';

这是上面生成的图像

Answer 1

SELECT COUNT(*)
FROM (
SELECT nmbr, post_id
     , DeliveryDate
     , DeliveryType
  FROM ( SELECT nmbr, post_id
              , 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

count() 函数 + derived table 和 Group - 使一切正常

count() function + derived table and Group - making it all work

mysql

count

sum

having-clause

derived-table