multiple top n aggregates query defined as a view (or function)? - sql

Question

I couldn't find a past question exactly like this problem. I have an orders table, containing a customer id, order date, and several numeric columns (how many of a particular item were ordered on that date). Removing some of the numberics, it looks like this:
customer_id date a b c d
0001 07/01/22 0 3 3 5
0001 07/12/22 12 0 50 0
0002 06/30/22 5 65 0 30
0002 07/20/22 1 0 19 2
0003 08/01/22 0 0 99 0
I need to sum each numeric column by customer_id, then return the top n customers for each column. Obviously that means a single customer may appear multiple times, once for each column. Assuming top 2, the desired output would look something like this:
column_ranked customer_id sum rank
'a' 001 12 1
'a' 002 6 2
'b' 002 65 1
'b 001 3 2
'c' 003 99 1
'c' 001 53 2
'd' 002 30 1
'd' 001 5 2
(this assumes no date range filter)
My first thought was a CTE to collapse the table into its per-customer sums, then a union from the CTE, with a limit n clause, once for each summed column. That works if the date range is hard-coded into the CTE .... but I want to define this as a view, so it can be called by users something like this:
SELECT * from top_customers_view WHERE date_range BETWEEN ( date1 and date2 )
How can I pass the date restriction down to the CTE? Or am I taking the wrong approach entirely? If a view isn't possible, can it be done as a function? (without using a costly cursor, that is.)

Answer 1

Since the date ranges clearly produce a massive number of combinations you cannot generate a view with them. You can write a query, however, as shown below:
with
p as (select cast ('2022-01-01' as date) as ds, cast ('2022-12-31' as date) as de),
a as (
select top 10 customer_id, 'a' as col, sum(a) as s
from t cross join p where date between ds and de
group by customer_id order by s desc
),
b as (
select top 10 customer_id, 'b' as col, sum(b) as s
from t cross join p where date between ds and de
group by customer_id order by s desc
),
c as (
select top 10 customer_id, 'c' as col, sum(b) as s
from t cross join p where date between ds and de
group by customer_id order by s desc
),
d as (
select top 10 customer_id, 'd' as col, sum(b) as s
from t cross join p where date between ds and de
group by customer_id order by s desc
)
select * from a
union all select * from b
union all select * from c
union all select * from d
order by customer_id, col, s desc
The date range is in the second line.
See db<>fiddle.
Alternatively, you could create a data warehousing solution, but it would require much more effort to make it work.