当一个为null时添加子查询

Question

SELECT (select sum(furniture.furniture_sales)
            from furniture
            where cp.userid = furniture.userid) + 
        (select sum(electronics.electronic_sales)
            from electronics
            where cp.userid = electronics.userid) as total  
FROM customer_profile cp

基本上，如果家具或电子产品中不存在用户标识，则“保持”将返回NULL。我能做什么，以便'total'将返回一个值而不是NULL？