How-to Join wordpress Query

I don’t understand exactly what you are trying to achieve. Can you explain more and give examples?

Even though the meta_values may be unique, there may be different posts for each one. Do you just want to show the first post for each meta_value?

You need to add a filter to the query to include the meta_value.

The code below should be close to what you want.

function mam_posts_fields ($fields) {
      global $mam_global_fields;
      // Make sure there is a leading comma
      if ($mam_global_fields) $fields .= (preg_match('/^(\s+)?,/',$mam_global_fields)) ? $mam_global_fields : ", $mam_global_fields";
      return $fields;
   }

add_filter('posts_fields','mam_posts_fields');
$mam_global_fields = 'meta_value';
query_posts('category_name=CATNAME&meta_key=METANAME&orderby=meta_value');
$mam_global_fields = '';  //Turn off filter
if (have_posts()) {
   $seen = array();
   while (have_posts()) {
      if (in_array($post->meta_value,$seen)) continue;
      $seen[] = $post->meta_value;
      // Rest of code to display a post
   }
} else {
   // Code for no posts found
}