MySQL Select查詢-用例何時或加入？

Question

我有一個簡單的Select語句，如下所示：

select p.ID as order_id,
   p.post_date,
   max(CASE WHEN pm.meta_key = '_billing_email' and p.ID = pm.post_id
                 THEN pm.meta_value END) as billing_email,
   max(CASE WHEN pm.meta_key = '_billing_first_name' and p.ID = pm.post_id
                 THEN pm.meta_value END) as _billing_first_name,
   max(CASE WHEN pm.meta_key = '_billing_last_name' and p.ID = pm.post_id
                 THEN pm.meta_value END) as _billing_last_name,
   max(CASE WHEN pm.meta_key = '_order_total' and p.ID = pm.post_id
                 THEN pm.meta_value END) as order_total,
   max(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 = '_paid_date' and p.ID = pm.post_id
                 THEN pm.meta_value END) as paid_date
  from wp_posts as p,
  wp_postmeta as pm
  where post_type = 'shop_order'
   and p.ID = pm.post_id
   and
   and post_status = 'wc-completed'
  group by p.ID

有人建議我應該使用JOIN語法-我假設使用的不是CASE WHEN。 我已經開始研究JOIN版本，但到目前為止，它似乎還不那么冗長-只是想知道這是否確實是最佳實踐，並且處理效率更高？

我是SQL的新手，我正在學習，所以很想看看如何使用JOIN重寫它的示例，因為我可能會以錯誤的方式進行操作。

Answer 1

在MySQL中，基本上有兩種數據透視方法。 您應該修復您的from子句以具有顯式join ：

from wp_posts p join
     wp_postmeta pm
     on p.ID = pm.post_id
where p.post_type = 'shop_order' and p.post_status = 'wc-completed'

您用於合並有關帖子數據的方法使用匯總。 join方法如下所示：

select p.*,
       pm_be.meta_value as billing_email,
       pm_fn.meta_value as billing_first_name,
from wp_posts p left join
     wp_postmeta pm_be
     on p.ID = pm_be.post_id and pm_be.meta_key = '_billing_email' left join
     wp_postmeta pm_fn
     on p.ID = pm_fn.post_id and pm_fn.meta_key = '_billing_first_name' left join
      . . .
where p.post_type = 'shop_order' and p.post_status = 'wc-completed'

如果性能是一個問題，那么值得嘗試兩種方法。 它們並不完全相同。

首先，聚合方法（您的方法）每個帖子僅產生一行，而與可能具有相同鍵值的值的數量無關。 您可以使用group_concat()而不是max()獲得所有值。

這是一個優點，因為join方法將為給定的鍵返回多個行，而這通常不是您想要的。 您可以使用group by解決它，但這會產生開銷。

假設索引設置正確， join方法通常會更快地從表中獲取少數列。 聚合方法的一個優點是，添加新鍵基本上不會增加​​開銷-本質上，聚合已經非常昂貴，以至於額外的max()或listagg()不會增加太多。

無論使用哪種方法，都應遵循一個簡單的規則： 切勿在FROM子句中使用逗號。 始終使用帶有ON子句的顯式JOIN語法。

Answer 2

嘗試使用它（使用連接）

   select p.ID as order_id,
   p.post_date,
   max(CASE WHEN pm.meta_key = '_billing_email' THEN pm.meta_value END) as billing_email,
   max(CASE WHEN pm.meta_key = '_billing_first_name' THEN pm.meta_value END) as _billing_first_name,
   max(CASE WHEN pm.meta_key = '_billing_last_name' THEN pm.meta_value END) as _billing_last_name,
   max(CASE WHEN pm.meta_key = '_order_total' THEN pm.meta_value END) as order_total,
   max(CASE WHEN pm.meta_key = '_order_tax' THEN pm.meta_value END) as order_tax,
   max(CASE WHEN pm.meta_key = '_paid_date' THEN pm.meta_value END) as paid_date
  from wp_posts as p join
  wp_postmeta as pm on (p.ID = pm.post_id)
  where post_type = 'shop_order'and 
   and post_status = 'wc-completed'
  group by p.ID

您可能需要用max（p.post_date）替換p.post_date

select p.ID as order_id,
   max(p.post_date),
   max(CASE WHEN pm.meta_key = '_billing_email' THEN pm.meta_value END) as billing_email,
   max(CASE WHEN pm.meta_key = '_billing_first_name' THEN pm.meta_value END) as _billing_first_name,
   max(CASE WHEN pm.meta_key = '_billing_last_name' THEN pm.meta_value END) as _billing_last_name,
   max(CASE WHEN pm.meta_key = '_order_total' THEN pm.meta_value END) as order_total,
   max(CASE WHEN pm.meta_key = '_order_tax' THEN pm.meta_value END) as order_tax,
   max(CASE WHEN pm.meta_key = '_paid_date' THEN pm.meta_value END) as paid_date
  from wp_posts as p join
  wp_postmeta as pm on (p.ID = pm.post_id)
  where post_type = 'shop_order'and 
   and post_status = 'wc-completed'
  group by p.ID