Mes NULL
(lecture: Clausula IN / NOT IN)
SELECT c.Cli_Id AS 'Cod. Cliente',
c.Cli_RazonSocial AS 'Razon Social/Nombre',
(SELECT SUM(v.Ventas_Total) FROM ventas v WHERE v.Ventas_CliId=c.Cli_Id AND YEAR(Ventas_Fecha)=2018) AS 'Venta Total Anual',
(SELECT SUM(v.Ventas_Total) FROM ventas v WHERE v.Ventas_CliId = c.Cli_Id AND Month(Ventas_Fecha)=02) AS 'Venta Total Mensual'
FROM clientes c
WHERE c.Cli_Id IN
(SELECT distinct(Ventas_CliId) AS 'Cod. Cliente' FROM ventas WHERE Ventas_Fecha>='2018-02-02' and Ventas_Fecha>='2018-03-01')
ORDER BY c.Cli_Id