How do I return a field in MySQL from a sub-query linked to the main query results?

Question

I'm having a bit of a problem with a query I'm using on a wordpress database. The query below is returning the correct and expected data for meta_value but my problem comes from the fact I don't get a field returned for the original post_id from the subquery , so I'm not able to link a specific meta_value with the original post_id - I may well need to restructure this but I'm a bit lost as to how to return this data associated with the meta_value it found.

SELECT meta_value
FROM wp_postmeta 
WHERE post_id IN (SELECT meta_value FROM wp_postmeta WHERE post_id IN ('1','2','3','4')) 
AND meta_key = '_wp_attached_file' 

Sample data

post_id    meta_key            meta_value
1          _thumbnail_id       2
2          _wp_attached_file   image.jpg

So as an example, given a list of 1 or more post_ids ('1'), I find the meta_value ('2') and look for another entry with a matching post_id ('2') and specified meta_key ('_wp_attached_file'), and I need to return both the meta_value ('image.jpeg') and the original post_id ('1')

Thanks in advance

Answer 1

Just use a self join:

select wp1.*, wp2.* 
from wp_postmeta wp1, wp_postmeta wp2 
where wp2.post_id in (1,2,3) 
and wp1.meta_value = wp2.post_id and wp2.meta_key = '_wp_attached_file';