Custom Select Query by meta key and cat id

Hi I have this custom query that get the expiry date of the post and orders post by that. How do I amend it to only list from a certain category id e.g. ’20’

$querystr = "
    SELECT wposts.*
    FROM $wpdb->posts wposts, $wpdb->postmeta wpostmeta
    WHERE wposts.ID = wpostmeta.post_id
    AND wpostmeta.meta_key = 'es_ape_expiry'
    ORDER BY wpostmeta.meta_value ASC
    ";

 $pageposts = $wpdb->get_results($querystr, OBJECT);

many thanks