how-to-get-a-meta-values-from-all-posts-in-a-special-category

meta-fields-from-all-posts.php

<?php
// http://wordpress.stackexchange.com/questions/64338/how-to-get-a-meta-value-from-all-post
// https://wordpress.org/support/topic/how-to-get-a-meta-values-from-all-posts-in-a-special-category
function get_meta_values( $key = '', $type = 'post', $status = 'publish' ) {
    if( empty( $key ) ) return;

    global $wpdb;

    $r = $wpdb->get_results( $wpdb->prepare( "
        SELECT p.ID, pm.meta_value FROM {$wpdb->postmeta} pm
           LEFT JOIN {$wpdb->posts} p ON p.ID = pm.post_id
           LEFT JOIN {$wpdb->term_relationships} tr ON p.ID = tr.object_id
           LEFT JOIN {$wpdb->term_taxonomy} tt ON tr.term_taxonomy_id = tt.term_taxonomy_id
        WHERE pm.meta_key = '%s' 
           AND p.post_status = '%s' 
           AND p.post_type = '%s'
           AND tt.taxonomy = '%s'
           AND tt.term_id = '%d'
    ", $key, $status, $type, 'product_cat', 38 ));

    foreach ( $r as $my_r ) {
        $metas[$my_r->ID] = $my_r->meta_value;
    }

    return array_unique($metas);
}


dump( get_meta_values('manufacturer', 'product') );