Q&A for Drupal developers and administrators
select t2.d_year, t2.d_month, Round(100.0 * t2.diff / t2.prev_month, 2) as Percentage from ( select t1.d_year, t1.d_month, count(t1.id) as d_count, LAG(count(t1.id)) OVER (ORDER BY t1.d_year, t1.d_month) AS prev_month, count(t1.id) - (LAG(count(t1.id)) OVER (ORDER BY t1.d_year, t1.d_month)) as diff from ( select id, YEAR(CreationDate) as d_year, MONTH(CreationDate) as d_month from posts ) t1 group by t1.d_month, t1.d_year ) t2 where Round(100.0 * t2.diff / t2.prev_month,2) < 0 order by t2.d_year, t2.d_month