Order by RAND

<?php
		if (have_posts()) :
		$categories = get_categories();
		foreach($categories as $category) {


		global $wpdb;
		$sql = "SELECT p.ID, p.post_title, p.post_name, pm.meta_value "
			."FROM ".$wpdb->posts." p
			INNER JOIN ".$wpdb->term_relationships." r ON ( p.ID = r.object_id )
			INNER JOIN ".$wpdb->postmeta." pm ON ( p.ID = pm.post_id AND pm.meta_key = 'thumbnail_url' )
			INNER JOIN ".$wpdb->term_taxonomy." t ON ( r.term_taxonomy_id = t.term_taxonomy_id ) 
			WHERE t.taxonomy = 'category' AND t.term_id IN (".$category->term_id.") AND p.post_status = 'publish'"
			. "AND p.post_type = 'post' ORDER BY p.ID DESC LIMIT 0,6";
		$posts = $wpdb->get_results($sql);
		?>