var custs = from c in db.T_Customer

                        join u in db.Sys_User on c.OwnerId equals u.Id into temp

                        from t in temp.DefaultIfEmpty()  //left join

                        where (c.PhoneNum1 == phone || c.PhoneNum2 == phone || c.PhoneNum3 == phone)

                        select new

                        {

                            CustName = string.IsNullOrEmpty(c.CustName) ? "匿名" : c.CustName,

                            UserName = t == null ? "暂无" : t.Name//这里主要第二个集合有可能为空。需要判断

                        };