Stock Level Stratification :Group Products by Inventory Levels into Four Category-Based Stock Tiers Intermediate

          with stocklevels as
  (select p.category,
          i.productid,
          sum(i.quantityavailable) as totalstock
   from inventory i
   join products p on i.productid = p.productid
   group by p.category,
            i.productid)
select category,
       productid,
       totalstock,
       ntile(4) over (partition by category
                      order by totalstock desc) as stocktier
from stocklevels;