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;