急急急！VB对Access数据库进行sum求和总是出错！

SQL = "select * from "

        SQL = SQL & "(SELECT 开单日期,业务类型,单号,sum(销售合计) as 欠货金额,null as 欠货归还金额 FROM 商店收支表 WHERE 业务类型= '欠货' and 顾客编号='" & txtCusnum.Text & "'and format(开单日期,'yyyy-mm') between '" & Format(start, "yyyy-mm") & "' and '" & Format(finish, "yyyy-mm") & "' and 是否作废='否' group by 业务类型,开单日期,单号,顾客编号,顾客姓名 "

        SQL = SQL & "union all SELECT 开单日期,业务类型,单号,null as 欠货金额,sum(销售合计) as 欠货归还金额 FROM 商店收支表 WHERE 业务类型= '欠货归还' and 顾客编号='" & txtCusnum.Text & "'and format(开单日期,'yyyy-mm') between '" & Format(start, "yyyy-mm") & "' and '" & Format(finish, "yyyy-mm") & "'and 是否作废='否' group by 业务类型,开单日期,单号,顾客编号,顾客姓名 )as t order by 开单日期,单号"

        Adodc1.ConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" & gFile & ";Mode=ReadWrite;Persist Security Info=False"

        Adodc1.RecordSource = SQL

        Adodc1.Refresh

代码如上，同样的一张表，第二行的sum求和函数显示正确，第三行的sum用debug输出后显示为？
求大神指点！！！！！