Interview question:How to get last 3 month aggregation at column level?

Question

This is the question i was being asked at Apple onsite interview and it blew my mind. Data is like this:

orderdate,unit_of_phone_sale

20190806,3000

2019070         


        

Answer 1

I think LEAD function can help here -

SELECT TO_CHAR(orderdate, 'YYYYMM') "DATE"
  ,unit_of_phone_sale M_1_Sale
  ,LEAD(unit_of_phone_sale,1,0) OVER(ORDER BY TO_CHAR(orderdate, 'YYYYMM') DESC) M_2_Sale
  ,LEAD(unit_of_phone_sale,2,0) OVER(ORDER BY TO_CHAR(orderdate, 'YYYYMM') DESC) M_3_Sale
  ,LEAD(unit_of_phone_sale,3,0) OVER(ORDER BY TO_CHAR(orderdate, 'YYYYMM') DESC) M_4_Sale
FROM table_sales

Here is the DB Fiddle