Group repeated OVER clauses together in Postgres

Question

Is it possible to somehow combine several repeated OVER clauses together in Postgres?

Example with a CASE ... END scenario:

   case
        when hhk = lag(hhk) over (order by hhk, product, dt)
        and product = lag(product) over (order by hhk, product, dt)
        then dt - lag(dt) over (order by hhk, product, dt)
        else null
    end as delta

The clause over (order by hhk, product, dt) is repeated three times. I'm looking for a way to somehow group them together, like the following (which of course doesn't work as such):

    case
        -- here we combine the two conditions into one
        when [hhk, product] = lag([hhk, product]) 
            over (order by hhk, product, dt)
        -- and here we somehow recall the clause
        then dt - lag(dt) over (my_clause)
        else null
    end as delta

Answer 1

You can define a window in the FROM clause. For example:

select v.*, row_number() over w
from (values (1, 2), (1, 3)) v(x, y)
     window w as (partition by x order by y)

In your specific example, I can speculate something like this:

select . . .
       (case when household_key = lag(household_key) over w and
                  product_id = lag(product_id) over w
        then dt - lag(dt) over w
        end) as delta
from . . .
     window w as (order by household_key, product_id, dt)