select mes
(select sum(unidades) from ventas where ventas.mes = meses.mes and producto = 'A') as ventas_A,
(select sum(unidades) from ventas where ventas.mes = meses.mes and producto = 'B') as ventas_B,
(select sum(unidades) from ventas where ventas.mes = meses.mes and producto = 'C') as ventas_C
from meses