Oracle SQL :LAG function

The LAG function provides access to a row at a given offset prior to the current position, and the LEAD function provides access to a row at a given offset after the current position.

Example: How to

Retrieve the quantity sold from the previous month ?

SELECT CALENDAR_MONTH_DESC, 
       SUM(quantity_sold), 
       lag(SUM(quantity_sold),1) OVER (ORDER BY CALENDAR_MONTH_DESC) "Previous Month" 
FROM 
       sales,
       TIMES
WHERE
       times.TIME_ID = sales.TIME_ID AND
       times.CALENDAR_YEAR = '2019'
GROUP BY CALENDAR_MONTH_DESC
ORDER BY CALENDAR_MONTH_DESC ASC