如何使用MySQL列别名进行计算？

Question

如何使用两个子查询中的列别名(lat和lng)来进行距离计算？我基本上要做的是使用经度和纬度值计算两个位置之间的距离.但不知怎的,我的别名在查询中不可用,为什么？

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

Answer 1

好的，您需要在这里做的是在不同的别名下连接同一个表（wp_postmeta）两次，以便您可以使用不同的“WHERE”条件。我没有你的表格，所以我无法测试这个，但这是你想要使用的方法：

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

我可能有一些语法错误，但我相信逻辑大致是正确的。让我知道这个是否奏效。