SQL LEFT JOIN 子查询别名
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SQL LEFT JOIN Subquery Alias
提问by CharleyXIV
I'm running this SQL query:
我正在运行这个 SQL 查询：
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
And I get this error:
我得到这个错误：
#1054 - Unknown column 'a.post_id' in 'on clause'.
#1054 -“on 子句”中的未知列“a.post_id”。
I think my code is pretty simple, but I can't make it right. What am I doing wrong?
我认为我的代码很简单，但我做对了。我究竟做错了什么？
回答by Mahmoud Gamal
You didn't select post_idin the subquery. You have to select it in the subquery like this:
您没有post_id在子查询中选择。您必须像这样在子查询中选择它：
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
回答by EJay
I recognize that the answer works and has been accepted but there is a much cleaner way to write that query. Tested on mysql and postgres.
我认识到答案有效并已被接受，但有一种更简洁的方式来编写该查询。在 mysql 和 postgres 上测试。
SELECT wpoi.order_id As No_Commande
FROM wp_woocommerce_order_items AS wpoi
LEFT JOIN wp_postmeta AS wpp ON wpoi.order_id = wpp.post_id
AND wpp.meta_key = '_shipping_first_name'
WHERE wpoi.order_id =2198
