WITH
t_12 AS (SELECT EXTRACT(MONTH FROM CAST(invoice_date AS date)) AS month, 
             SUM(total) AS sum_total_2012
             FROM invoice
             WHERE EXTRACT(YEAR FROM CAST(invoice_date AS timestamp)) = 2012
             GROUP BY month),
t_13 AS (SELECT EXTRACT(MONTH FROM CAST(invoice_date AS date)) AS month, 
             SUM(total) AS sum_total_2013
             FROM invoice
             WHERE EXTRACT(YEAR FROM CAST(invoice_date AS timestamp)) = 2013
             GROUP BY month)
SELECT t_12.month,
       sum_total_2012,
       sum_total_2013,
       ROUND((sum_total_2013 - sum_total_2012)/sum_total_2013*100) AS perc
FROM t_12 LEFT JOIN t_13 ON t_12.month=t_13.month
ORDER BY perc;