Query by Category Name and Custom Field Key Sorted by Key's Value This example selects posts that have a common category name and custom field key, then sorts the selection by the custom key's value. From http://codex.wordpress.org/Displaying_Posts_Using_a_Custom_Select_Query

query-cat-and-key.php

$querystr = "
	SELECT * FROM $wpdb->posts
	LEFT JOIN $wpdb->postmeta ON($wpdb->posts.ID = $wpdb->postmeta.post_id)
	LEFT JOIN $wpdb->term_relationships ON($wpdb->posts.ID = $wpdb->term_relationships.object_id)
	LEFT JOIN $wpdb->term_taxonomy ON($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id)
	LEFT JOIN $wpdb->terms ON($wpdb->term_taxonomy.term_id = $wpdb->terms.term_id)
	WHERE $wpdb->terms.name = 'slides'
	AND $wpdb->term_taxonomy.taxonomy = 'category'
	AND $wpdb->posts.post_status = 'publish'
	AND $wpdb->posts.post_type = 'post'
	AND $wpdb->postmeta.meta_key = 'order'
	ORDER BY $wpdb->postmeta.meta_value ASC
";