If meta_key exists then don’t select this post?
There are a few ways you could do it- NOT IN, NOT EXISTS, IS NULL. Which method is best depends on your actual query. SELECT * FROM $wpdb->posts LEFT JOIN $wpdb->postmeta ON $wpdb->posts.ID = $wpdb->postmeta.post_id AND $wpdb->postmeta.meta_key = ‘somekey’ WHERE $wpdb->postmeta.post_id IS NULL