Ms Access中的减号在2个不同的表中

Question

我在1个Access数据库中有2个不同的表。我尝试制作“限制” - “使用”并在文本框中显示。

        private void btEnter_Click(object sender, EventArgs e)
    {
            string Constr = "Provider=Microsoft.JET.OLEDB.4.0;Data Source = BankServer.mdb"; //select connection
            OleDbConnection Conn = new OleDbConnection(Constr);
            Conn.Open();
            string limit = "Select CardLimit from Server"; //get data from server
            string usage = "Select UseAmount from Record"; //get data from Record

            OleDbCommand cmd1 = new OleDbCommand(limit,Conn);
            OleDbDataReader read1 = cmd1.ExecuteReader();

            OleDbCommand cmd2 = new OleDbCommand(usage, Conn);
            OleDbDataReader read2 = cmd2.ExecuteReader();

answer = read1 - read2; ＆lt; ----需要这方面的帮助。

在ms.Access“CardLimit”和“UseAmount”中设置为数字类型。