如何在滞后函数中使用别名列

Question

我如何使用别名列sisa滞后于case语句

select i.item_name , i.item_code, DATE(ic.transaction_date), doc_no, qty,
case when count(item_name) over(partition by item_name) = 1
        then sum(qty) over(partition by doc_no)
    else 
        case when row_number() over(partition by item_code) = 1
            then first_value(qty) over(partition by item_code)
            else
                qty + lag(sisa)over (partition by item_code)
        end
end as sisa
from item_card_today ic
join item i on i.item_id = ic.item_id
where transaction_date 
between '2019-03-31 00:11:42'::timestamp and '2019-04-02 08:17:35'::timestamp
and i.item_code = 'CLDXMZM3NTDGSHCKPRFHBHTM'
order by item_code asc, item_name asc

我如何在代码中使用sisa？