如何使用MySQL列别名进行计算？ [英] How to use a MySQL column alias for calculations?

问题描述

如何使用两个子查询中的列别名（lat和lng）进行下面的距离计算？我基本上想做的是使用经度和纬度值计算两个位置之间的距离。但是为什么我的别名在查询中不可用，为什么呢？

How can I use my column alias (lat and lng) from the two subqueries to make the distance calcuation underneath? What I am basically trying to do is is to calculate the distance between two locations using longitude and latitude values. But somehow my aliases aren't usable in the query, why?

SELECT wp_posts.*,
(SELECT wp_postmeta.meta_value FROM wp_postmeta WHERE wp_postmeta.post_id = wp_posts.ID   AND wp_postmeta.meta_value LIKE '41.%') AS lat,
(SELECT wp_postmeta.meta_value FROM wp_postmeta WHERE wp_postmeta.post_id = wp_posts.ID AND wp_postmeta.meta_value LIKE '2.%') AS lng,
(3959 * acos( cos( radians(41.367682) ) * cos( radians( 'lat' ) ) * cos( radians('lng') -     radians(2.154077)) + sin(radians(41.367682)) * sin( radians('lat')))) AS distance
FROM wp_posts, wp_postmeta
WHERE wp_posts.ID = wp_postmeta.post_id AND wp_postmeta.meta_key = 'position' AND wp_posts.post_status = 'publish' AND wp_posts.post_type = 'page' AND wp_posts.post_date <  NOW()
GROUP BY ID
ORDER BY distance ASC

推荐答案

好，您需要做的是使用不同的别名将同一张表（wp_postmeta）连接两次，以便可以使用不同的 WHERE条件。我没有您的桌子，所以无法测试，但这是您要使用的方法：

OK, What you need to do here is join the same table (wp_postmeta) twice under different aliases so you can use different 'WHERE' conditions. I don't have your tables so I can't test this, but this is the approach you'll want to use:

SELECT wp_posts.*,
     (`alias_1`.meta_value) AS `lat`,
     (`alias_2`.meta_value) AS `lng`,
     (3959 * acos( cos( radians(41.367682) ) * cos( radians( `lat` ) ) * cos( radians( `lng` ) -     radians(2.154077)) + sin(radians(41.367682)) * sin( radians( `lat` )))) AS `distance`
FROM wp_posts
     LEFT JOIN `wp_postmeta` AS `alias_1` ON wp_posts.ID = alias_1.post_id
     LEFT JOIN `wp_postmeta` AS `alias_2` ON wp_posts.ID = alias_2.post_id
WHERE 
     wp_posts.post_status = 'publish' 
     AND wp_posts.post_type = 'page' 
     AND wp_posts.post_date <  NOW()
     AND `alias_1`.meta_key = 'position'
     AND `alias_1`.meta_value LIKE '41.%'
     AND `alias_2`.meta_key = 'position'
     AND `alias_2`.meta_value LIKE '2.%'
GROUP BY wp_posts.`ID`
ORDER BY `distance` ASC

我那里可能有一些语法错误，但我认为逻辑大致正确。让我知道这是否有效。

I may have some syntax errors in there, but I believe that the logic is roughly correct. Let me know if this works.

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