SELECT DISTINCT
    wpm.meta_value
FROM wp_postmeta wpm
JOIN wp_posts wp ON wpm.post_id = wp.ID
JOIN wp_postmeta wpm2 ON wpm.post_id = wpm2.post_id
WHERE
    /** specify that we only want items under subcat1 **/
    wpm.meta_key = 'subcat1' AND

    /** This is how we specify only to get values under commercial **/
    (wpm2.meta_key = 'category' AND wpm2.meta_value = 'commercial')