Help on mysql query (select attachment ID)

Hello. Most of the time I found answers searching the forum, but not this time. Can anyone help me on this query.

What am i doing wrong?

<?php
global $wpdb;
$attachment_id = $wpdb->get_var("
	SELECT
	$wpdb->posts.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
	FROM $wpdb->posts
	WHERE $wpdb->posts.post_type='attachment'
	AND $wpdb->posts.post_title='header'
	AND $wpdb->term_taxonomy.taxonomy = 'category'
	AND $wpdb->term_taxonomy.term_id = '5'
	ORDER BY post_date DESC LIMIT 1");
?><?php echo get_attachment_icon($attachment_id); ?>

What i’m trying to do is to put on the sidebar the latest post thumb (with “header” as image title) from an specific category. This is the first time i try to write my own query.

Thanks