SELECT
a.product_id
, a.cumulative as total_inventory
, a.cumulative - COALESCE(b.quantity,0) AS inventory_on_hand
FROM table1 a
JOIN
( SELECT MAX(id) AS max_id
FROM table1
GROUP BY product_id
) m ON (m.max_id = a.id)
LEFT JOIN
( SELECT product_id, SUM(quantity)
FROM table1
WHERE in_transit = 1
GROUP BY product_id
) b ON (a.product_id = b.product_id)