How to calculate a metric for current week/current year vs same week number/last year with a window function?

Question

I'm calculating some metrics and try to achieve this with a window function. First I calculated WEEKLY_PROFIT delta for current week/CURRENT year vs last week/CURRENT year.

How can I apply a window function to calculate WEEKLY_PROFIT for current week/CURRENT year vs same week number/LAST year?

SELECT TR_YR, TR_WEEK, WEEKLY_PROFIT,
COALESCE((WEEKLY_PROFIT/NULLIF(lag(WEEKLY_PROFIT, 1, 0) 
OVER (PARTITION BY TR_YR ORDER BY TR_WEEK), 0))-1, 0) AS DELTA_PROFIT_WEEKLY_VS_CY
FROM base_metrics
GROUP BY TR_YR, TR_WEEK

The table after calculating DELTA_PROFIT_WEEKLY_VS_CY

|TR_YR |TR_WEEK|WEEKLY_PROFIT|DELTA_PROFIT_WEEKLY_VS_CY|
| 2019 | 1     | 500.0       | 0.0                     |
| 2020 | 1     | 1000.0      | 0.0                     |      
| 2020 | 2     | 1500.0      | 0.5                     |                         
| 2020 | 3     | 700.0       | -0.53                   |  

               

Here is what I expect after calculating DELTA_PROFIT_WEEKLY_VS_LY (WEEKLY_PROFIT for current week/CURRENT year vs same week number/LAST year)

|TR_YR |TR_WEEK|WEEKLY_PROFIT|DELTA_PROFIT_WEEKLY_VS_CY|DELTA_PROFIT_WEEKLY_VS_LY|
| 2019 | 1     | 400.0       | 0.0                     | 0.0                     |
| 2020 | 1     | 1000.0      | 0.0                     | 1.5                     |
| 2020 | 2     | 1500.0      | 0.5                     | 0.0                     |
| 2020 | 3     | 700.0       | -0.53                   | 0.0                     |

Answer 1

Your code looks pretty good, except for the GROUP BY and PARTITION BY clause:

SELECT TR_YR, TR_WEEK, WEEKLY_PROFIT,
       COALESCE((WEEKLY_PROFIT/NULLIF(lag(WEEKLY_PROFIT, 1, 0) 
OVER (PARTITION BY TR_WEEK ORDER BY TR_YR), 0))-1, 0
               ) AS DELTA_PROFIT_WEEKLY_VS_CY
FROM base_metrics;

Answer 2

I feel this is easier done with a left join . You can handle nulls with coalesce if you later wish to

select a.tr_yr, 
       a.tr_week, 
       a.weekly_profit,
       a.weekly_profit/c.weekly_profit as profit_wow,
       a.weekly_profit/b.weekly_profit as profit_yoy
from t a
left join t b on a.tr_yr=b.tr_yr+1 and a.tr_week=b.tr_week
left join t c on a.tr_yr=c.tr_yr and a.tr_week=c.tr_week+1
order by a.tr_yr, a.tr_week;