http://forum.wmonline.com.br/topic/195799-somar-dados-de-uma-coluna/

http://postgresql.nabble.com/Somar-2-Colunas-de-diferentes-tabelas-td2031287.html

https://www.google.com.br/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=postgres%20fazer%20soma%20de%20colunas

http://www.postgresqltutorial.com/postgresql-inner-join/

http://www.guj.com.br/java/266706-somar-valor-agrupado-postgresql

Estudo
https://developers.google.com/terms/
https://developers.google.com/chart/terms#license