A utility function for WordPress to retrieve metadata from post to display in a list or drop-down

get_postmeta_tuples.php

<?php
/**
 * Retrive the tuples of metadata in a given set of post-types.
 *
 * @param $metakeys string|array - The metakeys to be the tuple elements
 * @param $post_types string|array optional - The post-types to limit the query on. Defaults to the current post-type
 *
 *  <?php
 *  $cities = get_postmeta_tuples( array( 'city', 'state' ), 'chapters' );
 *  ?>
 *  <h3>Cities with Local Chapters</h3>
 *  <ul> 
 *  <?php foreach( $cities as $city ): ?>
 *    <li><?php echo $city->city, ', ', $city->state; ?></li>
 *  <?php endforeach; ?>
 *  </ul>
 */
function get_postmeta_tuples( $metakeys, $post_types=null ) {
  global $wpdb, $post;

  // Prepare $post_type if not provided by caller
  if( is_null($post_types) ) {
    if( !empty($post->post_type) ) {
      $post_types = array( $post->post_type );

    } else {
      return array(); // Return no results
    }
  }

  $post_types = (array) $post_types;
  $metakeys = (array) $metakeys;

  // ensure we have metakeys to do the query on
  if( count($metakeys)==0 ) {
    return array(); // Return no results
  }


  // build the query
  $select = "SELECT ";
  $select_vals = array();
  $from = "\nFROM {$wpdb->posts} as posts";
  $join = "";
  $where = "\nWHERE posts.post_status = 'publish' AND posts.post_type IN ";
  $groupby = "\nGROUP BY ";
  $groupby_vals = array();

  foreach( $metakeys as $metakey ) {
    $metakey = $wpdb->escape($metakey);
    $select_vals[] = "{$metakey}meta.meta_value as {$metakey}";
    $groupby_vals[] = $metakey;
    $join .= "\nJOIN {$wpdb->postmeta} as {$metakey}meta on {$metakey}meta.post_id = posts.ID AND {$metakey}meta.meta_key = '{$metakey}'";
  }
  $select .= implode( ', ', $select_vals );
  $groupby .= implode( ', ', $groupby_vals );

  foreach( $post_types as $k => $post_type ) {
    $post_type = $wpdb->escape( $post_type );
    $post_types[$k] = $post_type;
  }
  $where .= "('".(implode("','", $post_types))."')";

  return $wpdb->get_results( $select . $from . $join . $where . $groupby );
}