LAG and LEAD in SQL Server: the previous row

LAG looks at the previous row in the order you set. LEAD looks forward. That is month-over-month without joining the table to itself.

Month-over-month revenue

WITH m AS (
  SELECT CONVERT(char(7), order_date, 120) AS month,
         SUM(total_amount) AS revenue
  FROM Orders
  GROUP BY CONVERT(char(7), order_date, 120)
)
SELECT month, revenue,
       LAG(revenue) OVER (ORDER BY month) AS prev_month,
       revenue - LAG(revenue) OVER (ORDER BY month) AS change
FROM m
ORDER BY month;

The first month has no previous row, so LAG is NULL. Wrap it in ISNULL if the sheet cannot stand a blank.