MySQL Select Query - 使用案例時還是加入？

Question

我有一個簡單的SELECT語句如下：

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 

它已經建議我應該使用JOIN語法 - 我認爲CASE WHEN代替。我已經開始研究一個JOIN版本，但目前爲止似乎並沒有那麼冗長 - 只是想知道這是否是最佳實踐並且更有效地處理等等？

我是新來SQL和學習，因爲我很想看到如何使用JOIN重寫這些示例，因爲我可能會以這種錯誤的方式進行。

Answer 1

基本上有在旋轉數據兩種方法來代替p.post_date MySQL的。您應該解決您的from條款有明確的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' 

您的數據圍繞柱結合方法使用聚合。該join方法是這樣的：

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，那麼它是值得嘗試的兩種方法。他們不完全一樣。

首先，聚合方法（您的方法）每個帖子只生成一行，而不管可能具有相同鍵值的值的數量。您可以使用group_concat()而不是max()來獲取所有值。

這是一個優點，因爲join方法會爲給定的鍵返回多行，而這通常不是您想要的。您可以使用group by解決該問題，但會產生開銷。

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

無論採用哪種方法，都應該遵循一條簡單的規則：從不在FROM子句中使用逗號。 總是使用明確的JOIN語法與ON子句。

Answer 2

嘗試使用這個（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 

您可能需要最多（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