Alter SQL query to return posts with unique custom field value, no duplicate values

Question

I currently have a list of posts with the custom field "Name," with several posts having the same value for "Name."

I would like to retrieve a list of posts that all have unique values for "Name." For example, Post 1 and Post 2 can't both be "Michael."

Is there a way to do this by altering the SQL query directly, before it's executed? I am currently using the function below (limit 5 posts and ordered randomly) to alter the query.

function alter_the_query( $request ) {
        $dummy_query = new WP_Query();  // the query isn't run if we don't pass any query vars
        $dummy_query->parse_query( $request );

        if ($dummy_query->is_category()) {
        $request['posts_per_page'] = 5;
                $request['orderby'] = 'rand';
        }
        return $request;
}

I am trying to avoid using SQL directly for sake of compatibility with future versions of Wordpress, but if that's the only way then I'm still open to those suggestions.

Answer 1

If you have several posts with the same value, but you want only the most recent ones with any value, as long as values don't get repeated, you'r best bet might be running two loops on the same query:

/* Fetch all possible values - use this to order the 
results and define which get selected */
$valueids = new WP_Query(array(
posts_per_page => -1,
meta_key => 'key',
));

/* First loop, to build an array of ids with single values */
$values = array();
$ids = array();
while ($valueids->have_posts()) : $valueids->the_post();
    $val = get_post_custom_values('key');
        if(!in_array($val,$values) {
            $values[] = $val;
            $ids[] = $post-ID;
        }
endwhile;

$valueids->rewind_posts();
$valueids->set( 'posts_per_page', 10 ) /* loop only for the ones you want */

/* Second loop, leaving out duplicated entries */
while ($valueids->have_posts()) : $valueids->the_post();
    if(in_array($post->ID,$ids) {
            // do you thing here
        }
endwhile;

ps: this is untested, but it should work properly.