If you can explain it in SQL, you can query for it! There are three places where we want to change the default query:
SELECT wp_posts.*
FROM wp_posts
INNER JOIN wp_postmeta ON (wp_posts.ID = wp_postmeta.post_id)
WHERE 1=1
AND wp_posts.post_type="post"
AND (wp_posts.post_status="publish")
AND wp_postmeta.meta_key = 'startDate'
AND CAST(wp_postmeta.meta_value AS CHAR) < '2011-03-23'
GROUP BY wp_posts.ID
ORDER BY wp_postmeta.meta_value DESC
LIMIT 0, 10
- The join should be a left join
- The where-clause
- The order
The join and the where-clause are added via the _get_meta_sql() function. The output is filtered, so we can hook into it:
add_filter( 'get_meta_sql', 'wpse12814_get_meta_sql' );
function wpse12814_get_meta_sql( $meta_sql )
{
// Move the `meta_key` comparison in the join so it can handle posts without this meta_key
$meta_sql['join'] = " LEFT JOIN wp_postmeta ON (wp_posts.ID = wp_postmeta.post_id AND wp_postmeta.meta_key = 'startDate') ";
$meta_sql['where'] = " AND (wp_postmeta.meta_value IS NULL OR wp_postmeta.meta_value < '" . date('Y-m-d') . "')";
return $meta_sql;
}
The order clause is filtered through posts_orderby:
add_filter( 'posts_orderby', 'wpse12814_posts_orderby' );
function wpse12814_posts_orderby( $orderby )
{
$orderby = 'COALESCE(wp_postmeta.meta_value, wp_posts.post_date) ASC';
return $orderby;
}
This gives us the following SQL query:
SELECT wp_posts.*
FROM wp_posts
LEFT JOIN wp_postmeta ON (wp_posts.ID = wp_postmeta.post_id AND wp_postmeta.meta_key = 'startDate')
WHERE 1=1
AND wp_posts.post_type="post"
AND (wp_posts.post_status="publish")
AND (wp_postmeta.meta_value IS NULL OR wp_postmeta.meta_value < '2011-03-23')
GROUP BY wp_posts.ID
ORDER BY COALESCE(wp_postmeta.meta_value, wp_posts.post_date) ASC
LIMIT 0, 10
Remember to unhook the filters after you did your query, otherwise you will mess up other queries too. And if possible you should not call query_posts() yourself, but modify the main post query that is done by WordPress while setting up the page.