这将为您提供所有没有元键fizzbizz的东西。自定义循环部分，我取消了directly from the codex.

$pageposts = $wpdb->get_results("
    SELECT * FROM wp_posts p 
    LEFT JOIN wp_postmeta m 
    ON p.ID = m.post_id 
    WHERE m.meta_key <> \'fizzbizz\'
    OR m.metakey IS NULL
    ORDER BY p.post_date DESC;
");

if ($pageposts):
    global $post;
    foreach ($pageposts as $post): 
    setup_postdata($post);
// now you are in the loop, use the_title() or whatever

这将在有键的地方获取所有内容，但值为“foobar”

$pageposts = $wpdb->get_results("
    SELECT * FROM wp_posts p 
    JOIN wp_postmeta m 
    ON p.ID = m.post_id 
    WHERE m.meta_key = \'fizzbizz\' 
    AND m.meta_value = \'foobar\' 
    ORDER BY p.post_date DESC;
");

最后一个很棘手，我不太确定。。。

编辑

修复了第一个查询，因为this question.