Days Since the Last Visit :Calculate Days Since Each Customer's Previous Order Intermediate

          select customerid,
       orderid,
       orderdate,
       lag(orderdate) over (partition by customerid
                            order by orderdate) as previous_orderdate,
                           orderdate::date - lag(orderdate::date) over (partition by customerid
                                                                        order by orderdate) as days_since_last_order
from orders
order by customerid,
         orderdate;