Query for a list of WordPress posts by category, but only those with a postmeta->meta_key='Price' that's >0 and NOT NULL.

posts-by-category-with-price.php

<?php
/*

  posts-by-category-with-price.php

  Query for a list of WordPress posts by category, 
  but only those with a postmeta->meta_key='Price' that's >0 and NOT NULL.
  
  Author: Mike Schinkel (http://mikeschinkel.com)

  Just drop this example into the root of your website and call directly to see it work.
  Use the class in your theme's functions.php file, or a .php file of a plugin you are writing.

  In Answer To: http://wordpress.stackexchange.com/questions/3291/

*/
include "../wp-load.php";

$category = PostsByCategoryWithPrice::get_category(0);
$posts = PostsByCategoryWithPrice::query();
$column=0;
foreach($posts as $post) {
  if ($column++>6)
    $column = 1;
  if ($category->id != $post->category_id) {
    $category = PostsByCategoryWithPrice::get_category($post); ?>
    <h2 style="clear:both;">
      <a href="<?php echo get_category_link($category->id); ?>">
        <?php echo($category->name); ?></a>
    </h2><?php
  }?>
  <div class="span-1 thumb20<?php echo ($count==6 ? ' last' : ''); ?>">
    <a class="thumb" href="<?php the_permalink(); ?>" rel="bookmark" 
       title="Link to <?php the_title(); ?>">
      <?php echo the_post_thumbnail( 'thumbnail' ); ?>
    </a>
  </div><?php
}?>
<hr class="gallery-grid" />

<?php
class PostsByCategoryWithPrice {
  static $hooks = array();
  private static function push_action($hook,$callable,$priority=10,$params=1) {
    self::$hooks[$hook] = $callable;
    add_action($hook,$callable,$priority,$params);
  }
  private static function pop_action($count=1) {
    for($i=$count; $i>0; $i--) {
      $hook = end(array_keys(self::$hooks));
      $callable = array_pop(self::$hooks);
      remove_action($hook,$callable);
    }
  }
  static function get_category($post=false) {
    if (!$post)
      $category = (object)array('id'=>false);
    else
      $category = (object)array(
        'id'    => $post->category_id,
        'name'  => $post->category_name,
        'slug'  => $post->category_slug,
        );
    return $category;
  }
  static function query() {
    self::push_action('posts_fields',array(__CLASS__,'posts_fields'));
    self::push_action('posts_join',array(__CLASS__,'posts_join'));
    self::push_action('posts_where',array(__CLASS__,'posts_where'));
    self::push_action('posts_orderby',array(__CLASS__,'posts_orderby'));
    $query = new WP_Query("posts_per_page=-1");
    self::pop_action(4);
    return $query->posts;
  }
  static function posts_fields($fields) {
    global $wpdb;
    $fields .= ",{$wpdb->terms}.term_id AS category_id,
                 {$wpdb->terms}.name AS category_name,
                 {$wpdb->terms}.slug AS category_slug";
    return $fields;
  }
  static function posts_join($join) {
    global $wpdb;
    $join .=<<<SQL
INNER JOIN {$wpdb->postmeta} 
        ON {$wpdb->postmeta}.post_id={$wpdb->posts}.ID 
       AND {$wpdb->postmeta}.meta_key='price'
INNER JOIN {$wpdb->term_relationships} 
        ON {$wpdb->term_relationships}.object_id={$wpdb->posts}.ID
INNER JOIN {$wpdb->term_taxonomy} 
        ON {$wpdb->term_taxonomy}.term_taxonomy_id={$wpdb->term_relationships}.term_taxonomy_id
INNER JOIN {$wpdb->terms} 
        ON {$wpdb->term_taxonomy}.term_id={$wpdb->terms}.term_id
SQL;
    return $join;
  }
  static function posts_where($where) {
    global $wpdb;
    $where .="  AND {$wpdb->term_taxonomy}.taxonomy='category' 
                AND IFNULL({$wpdb->postmeta}.meta_value,'0')!='0' ";
    return $where;
  }
  static function posts_orderby($orderby) {
    global $wpdb;
    $orderby = " {$wpdb->term_taxonomy}.name,
                 {$wpdb->posts}.post_title ASC ";
    return $orderby;
  }
}