need help in implementing a scenario - response (1) by dnoeth

It's easy to get the previous value with OLAP-functions:

select cust_id, date,
   min(Closing_Inventory) 
   over (partition by cust_id 
         order by date 
         rows between 1 preceding and 1 preceding) as Opening_Inventory
from tab
qualify Opening_Inventory is not null

Simply use this query in a UPDATE FROM.