WITH

m12 AS (SELECT EXTRACT(MONTH FROM CAST(invoice_date AS timestamp)) AS month,

     SUM(total) AS sum_total_2012

     FROM invoice

     WHERE EXTRACT(YEAR FROM CAST(invoice_date AS timestamp))=2012

     GROUP BY month),

     

m13 AS (SELECT EXTRACT(MONTH FROM CAST(invoice_date AS timestamp)) AS month,

     SUM(total) AS sum_total_2013

     FROM invoice

     WHERE EXTRACT(YEAR FROM CAST(invoice_date AS timestamp))=2013  

     GROUP BY month)

SELECT m12.month,

sum_total_2012,

sum_total_2013,

ROUND(((m13.sum_total_2013 - m12.sum_total_2012)/m13.sum_total_2013)*100 ) as perc

FROM m12 FULL JOIN m13 ON m12.month=m13.month

ORDER BY month