SELECT 101 as item_num, 22 as sub_inv, 'A1' as lot_num, 10 as quantity, date '2019-09-29' as lot_expiry_date from dual union all

SELECT 101 as item_num, 22 as sub_inv, 'A2' as lot_num, 14 as quantity, date '2019-09-28' from dual

)

select i.*,

(case when running_quantity < 20 then quantity

when running_quantity - quantity < 20 then 20 + quantity - running_quantity

else 0

end) as qty_fetched

from (select i.*,

sum(quantity) over (partition by item_num, sub_inv order by lot_expiry_date) as running_quantity

from inventory i

where item_num = 101 and sub_inv = 22

) i