Month-on-month growth

With last month's figure on the same row, growth is ordinary arithmetic:

SELECT month, revenue,
       revenue - LAG(revenue) OVER (ORDER BY month) AS change,
       100.0 * (revenue - LAG(revenue) OVER (ORDER BY month))
             / LAG(revenue) OVER (ORDER BY month) AS growth_pct
FROM monthly

growth_pct of 12.5 means revenue grew 12.5% on the month before; −5 means it fell 5%.

Writing LAG(revenue) OVER (ORDER BY month) three times is clumsy. Put it in a CTE once, as prev_revenue, and then the growth sum reads like a spreadsheet formula:

100.0 * (revenue - prev_revenue) / prev_revenue

Use 100.0 rather than 100. In some databases (SQL Server, PostgreSQL) dividing two whole numbers throws away the decimals.

Careful with gaps: LAG takes the previous row, not the previous month. If a month has no sales at all it has no row, and LAG reaches back to the month before that. Brightwell sells something every month, so it's safe here, but check before relying on it.