How to use a MySQL column alias for calculations?

问题:

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

回答1:

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

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

回答2:

Once you've created your alias (which you should add quotes before and after) you should not reference it as a string... subsequent references to the alias should be enclosed in backticks:

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

http://dev.mysql.com/doc/refman/5.0/en/problems-with-alias.html