Problem with the Group by help?

What you should do is apply SUM to CASE , having them separated the information is disaggregated. I also added O_CHAR(BOLETA.FECHA_BOLETA,'MM/YYYY') to SELECT but can not properly see how the GROUP BY

SELECT  VENDEDOR.ID_VENDEDOR,
        TO_CHAR(BOLETA.FECHA_BOLETA,'MM/YYYY') 'Fecha',
        SUM(CASE WHEN TO_CHAR(BOLETA.FECHA_BOLETA,'MM/YYYY')='05/2014' THEN COMISION_VENTAS.VALOR_COMISION_BOLETA
            ELSE  0
        END) "COMISION"
        FROM VENDEDOR
        JOIN BOLETA 
            ON BOLETA.ID_VENDEDOR=VENDEDOR.ID_VENDEDOR
        JOIN COMISION_VENTAS
            ON BOLETA.NRO_BOLETA=COMISION_VENTAS.NRO_BOLETA
        GROUP BY 
            VENDEDOR.ID_VENDEDOR, TO_CHAR(BOLETA.FECHA_BOLETA,'MM/YYYY');

use group also in the commission field: link

mysql> SELECT year, SUM(profit) AS profit
       FROM sales
       GROUP BY year ASC WITH ROLLUP;