Mysql, select value of one cell depending on contents of another cell - mysql

Question

My current code
SELECT post_id, ( 3959 * ACOS( COS( RADIANS( 34.09 ) ) * COS( RADIANS( lat ) ) * COS( RADIANS( lng ) - RADIANS( -117.55 ) ) + SIN( RADIANS( 34.09 ) ) * SIN( RADIANS( lat ) ) ) ) AS distance FROM wp_postmeta WHERE `meta_key` LIKE '%location_l%' HAVING distance < 2500 ORDER BY distance LIMIT 0 , 20
Here are two sample row of data (meta_key is not always long or lat):
post_id = 123
meta_key = location_longitude
meta_value = -119.890000
post_id = 123
meta_key = location_latitude
meta_value = 42.170000
How do I modify my query to replace 'lat' and 'lng' in my original query to be the contents of the meta_value listed above? Something like this?
select meta_value where meta_key = location_latitude

Answer 1

If you GROUP BY post_id, you should be able to use MAX() aggregates in place of lat,lng, surrounding CASE statements which determine whether the current row is latitude or longitude. The others will be NULL, and therefore eliminated by the aggregate.
I think this ought to work so you won't need any subselects.
SELECT
post_id,
( 3959 * ACOS( COS( RADIANS( 34.09 ) ) * COS( RADIANS( MAX(CASE WHEN meta_key='location_latitude' THEN meta_value ELSE NULL END) ) ) * COS( RADIANS( MAX(CASE WHEN meta_key='location_longitude' THEN meta_value ELSE NULL END) ) - RADIANS( -117.55 ) ) + SIN( RADIANS( 34.09 ) ) * SIN( RADIANS( MAX(CASE WHEN meta_key='location_latitude' THEN meta_value ELSE NULL END) ) ) ) ) AS distance
FROM wp_postmeta
WHERE `meta_key` IN ('location_latitude','location_longitude')
GROUP BY post_id
HAVING distance < 2500
ORDER BY distance
LIMIT 0 , 20
The moving parts here are:
MAX(CASE WHEN meta_key='location_latitude' THEN meta_value ELSE NULL END)
This translates as: If this row's meta_key is 'location_latitude', return the meta_value, otherwise return NULL. We expect then that since two rows are returned for the post_id (lat,lng), the MAX() value returned above is always the non-null one -- the correct latitude or longitude value from meta_value.

Answer 2

You need to join the table to itself (this is common in databases with EAV structure):
SELECT
post_id,
distance
FROM
( SELECT
lng.post_id,
( 3959 * ACOS( COS( RADIANS( 34.09 ) )
* COS( RADIANS( lat.meta_value ) )
* COS( RADIANS( lng.meta_value )
- RADIANS( -117.55 ) )
+ SIN( RADIANS( 34.09 ) )
* SIN( RADIANS( lat.meta_value ) )
)
) AS distance
FROM wp_postmeta AS lng
JOIN wp_postmeta AS lat
ON lat.post_id = lng.post_id
WHERE lng.meta_key = 'location_longitude'
AND lat.meta_key = 'location_latitude'
) AS tmp
WHERE distance < 2500
ORDER BY distance
LIMIT 0 , 20 ;

Answer 3

One approach is to replace the reference to the table (i.e. FROM wp_postmeta) in your query with an inline view, something like this:
FROM
( SELECT plat.post_id
, plat.meta_value AS lat
, plng.meta_value AS lng
FROM wp_postmeta plat
JOIN wp_postmeta plng
ON plat.post_id = plng.post_id
AND plat.meta_key = 'location_latitude'
AND plng.meta_ley = 'location_longitude'
) wpm
(NOTE: this assumes that there is only one row for each post_id for each of the two meta_key values of interest...)
This uses an "inline view" to combine the latitude and longitude values for each post_id into a single row. (You can run just the query for the inline view to confirm that its returning the results you expect.) We can use an inline view in place of a table reference, in more recent versions of MySQL (version >= 5.0).

Answer 4

SELECT a.post_id,
( 3959 * ACOS( COS( RADIANS( 34.09 ) ) * COS( RADIANS( lat.meta_value ) ) * COS( RADIANS( lng.meta_value ) - RADIANS( -117.55 ) ) + SIN( RADIANS( 34.09 ) ) * SIN( RADIANS( lat.meta_value ) ) ) ) AS distance
FROM wp_postmeta lat, wp_postmeta lng
WHERE lat.post_id = lng.postID
AND lat.meta_key = 'location_latitude'
AND lng.meta_key = 'location_longitude'
HAVING distance < 2500
ORDER BY distance LIMIT 0 , 20