Percent change with grouping variables

I have four grouping variables Month, State,County, City. In addition I have the metric column sales which can be null I would like to calculate the percent change of sales per month for each City. My solution would have the same grouping but with the sales column replaced by percent change for each month in calendar year 2019. Any help with a solution is appreciated.

Answer

You can use window functions:

select month, state, city, sales,
       lag(sales) over (partition by state, city order by month) as prev_month,
       (-1 + sales / lag(sales) over (partition by state, city order by month)) as change_ratio
from t;