SQL LEFT JOIN子查询别名 [英] SQL LEFT JOIN Subquery Alias

问题描述

我正在运行此SQL查询:

I'm running this SQL query:

SELECT wp_woocommerce_order_items.order_id As No_Commande
FROM  wp_woocommerce_order_items
LEFT JOIN 
    (
        SELECT meta_value As Prenom
        FROM wp_postmeta
        WHERE meta_key = '_shipping_first_name'
    ) AS a
ON wp_woocommerce_order_items.order_id = a.post_id
WHERE  wp_woocommerce_order_items.order_id =2198

我收到此错误:

#1054-"on子句"中的未知列"a.post_id".

#1054 - Unknown column 'a.post_id' in 'on clause'.

我认为我的代码很简单，但是我做错了.我究竟做错了什么?

I think my code is pretty simple, but I can't make it right. What am I doing wrong?

推荐答案

您没有在子查询中选择post_id.您必须像下面这样在子查询中选择它:

You didn't select post_id in the subquery. You have to select it in the subquery like this:

SELECT wp_woocommerce_order_items.order_id As No_Commande
FROM  wp_woocommerce_order_items
LEFT JOIN 
    (
        SELECT meta_value As Prenom, post_id  -- <----- this
        FROM wp_postmeta
        WHERE meta_key = '_shipping_first_name'
    ) AS a
ON wp_woocommerce_order_items.order_id = a.post_id
WHERE  wp_woocommerce_order_items.order_id =2198 

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