Question

我有一个简单的Select语句如下：

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

有人建议我应该使用JOIN语法 - 我假设代替CASE WHEN。我已经开始研究JOIN版本了，但到目前为止它看起来并不那么冗长 - 只是想知道这是否确实是最佳实践并且处理效率更高？

我是新手学习SQL并学习，所以我很乐意看到如何使用JOIN重写这个例子，因为我可能会以错误的方式解决这个问题。

Answer 1

在MySQL中基本上有两种旋转数据的方法。您应该修复from子句以明确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'

组合帖子数据的方法使用聚合。 join方法如下：

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'

如果性能是一个问题，那么值得尝试这两种方法。它们并不完全相同。

首先，聚合方法（您的方法）每个帖子只生成一行，而不管可能具有相同键值的值的数量。您可以使用group_concat()而不是max()来获取所有值。

这是一个优点，因为join方法会为给定的密钥返回多行，而这通常不是您想要的。你可以使用group by绕过它，但这会产生开销。

假设索引设置正确，join方法通常会更快地从表中获取少量列。聚合方法的一个优点是添加新密钥基本上不会增加任何开销 - 实际上聚合已经非常昂贵，以至于额外的max()或listagg()不会增加很多。

无论采用哪种方法，都应该遵循一条简单的规则：从不在FROM子句中使用逗号。始终使用带有JOIN子句的显式ON语法。

Answer 2

尝试使用此功能（使用加入）

   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

你可能需要用max（p.post_date）替换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

MySQL选择查询 - 用例何时加入？

2 个答案: