how to show posts that are missing a meta_value

Question

How can I build a query to find posts that DO NOT contain a certain meta key or meta value?

for example:

query_posts( array( 
    'meta_query' => array(
        array( 'key' => 'feature', 'value' => '_wp_zero_value', 'compare' => '!=' )         )
) );

Answer 1

Getting posts without a certain meta key is a little tricky, namely due to the database design and the nature of SQL joins.

AFAIK, the most efficient way would be to actually grab the post IDs that do have the meta key, and then exclude them from your query.

// get all post IDs that *have* 'meta_key' with a non-empty value
$posts = $wpdb->get_col( "SELECT post_id FROM $wpdb->postmeta WHERE meta_key = 'my_key' AND meta_value != ''" );

// get all posts *excluding* the ones we just found
query_posts( array( 'post__not_in' => $posts ) );

Answer 2

$args = array(
    'meta_query' => array(
        'relation' => 'OR', //default AND
        array(
            'key' => 'feature',
            'compare' => 'NOT EXISTS'
        ),
        array(
            'key' => 'feature',
            'value' => '_wp_zero_value',
            'compare' => '='
        )
    ));

$the_query = new WP_Query($args);
var_dump($the_query);

Read further : Wordpress Doc

NB: before the wordpress 3.9 there is a bug So this is the work around

array(
       'key' => 'feature',
       'compare' => 'NOT EXISTS',
       'value' => '' //add this empty value check for check NOT EXISTS
      ),