Group by a custom value field wordpress posts - php

Question

I have several posts with a custom field named "series". I want to group all posts by this custom field and below of this i want to list all posts which have not this custom field.
First i wanted to get grouped custom field value and then to be able to query again for all posts with this custom value key and value. But even trying to get the unique custom values does not work.
What i tried is this:
<?php
function query_group_by_filter($groupby){
global $wpdb;
return $wpdb->postmeta . '.meta_key = "series"';
}
?>
<?php add_filter('posts_groupby', 'query_group_by_filter'); ?>
<?php $states = new WP_Query(array(
'meta_key' => 'series',
'ignore_sticky_posts' => 1
));
?>
<?php remove_filter('posts_groupby', 'query_group_by_filter'); ?>
<ul>
<?php
while ( $states->have_posts() ) : $states->the_post();
$mykey_values = get_post_custom_values( 'series' );
foreach ( $mykey_values as $key => $value ) {
echo "<li>$key => $value ( 'series' )</li>";
}
endwhile;
?>
</ul>
Whats wrong with this code?

Answer 1

WP_Meta_Query
All posts with custom value:
<?php
$args = array(
'post_type' => 'my-post-type',
'post_status' => 'publish',
'posts_per_page' => -1,
'meta_query' => array(
'relation' => 'AND',
array(
'key' => 'series',
'value' => 'my-val',
'compare' => '='
),
array(
'key' => 'series',
'value' => '',
'compare' => '!='
)
)
);
$the_query = new WP_Query( $args );
if ( $the_query->have_posts() ) : ?>
<ul>
<?php while ( $the_query->have_posts() ) : $the_query->the_post(); ?>
<li id="post-<?php the_ID(); ?>">
<?php the_title(); ?>
</li>
<?php
endwhile;
wp_reset_postdata(); ?>
</ul>
<?php endif; ?>
And without values:
<?php
$args = array(
'post_type' => 'my-post-type',
'post_status' => 'publish',
'posts_per_page' => -1,
'meta_query' => array(
array(
'key' => 'series',
'value' => '',
'compare' => '='
)
)
);
$the_query = new WP_Query( $args );
if ( $the_query->have_posts() ) : ?>
<ul>
<?php while ( $the_query->have_posts() ) : $the_query->the_post(); ?>
<li id="post-<?php the_ID(); ?>">
<?php the_title(); ?>
</li>
<?php
endwhile;
wp_reset_postdata(); ?>
</ul>
<?php endif; ?>