如何计算余额

Public Sub calYuE()
    Dim Db As ADODB.Connection
    Dim x As Currency    '上一条记录的余额
    Set Db = New ADODB.Connection
    Set rst = New ADODB.Recordset

    If MsgBox("你确定要计算余额吗?", vbYesNo + vbInformation, "提示") = vbYes Then
        '指定rst的连结数据库为目前打开的数据库
        rst.ActiveConnection = CurrentProject.Connection
        rst.Open "会计分录簿", , adOpenKeyset, adLockOptimistic, adCmdTable
        rst.MoveFirst
        x = 0
        Do Until rst.EOF
            If IsNull(rst!借方金额) Then    '即借方金额为0
                rst!余额 = x - rst!贷方金额
            Else    '即贷方金额为0
                rst!余额 = x + rst!借方金额
            End If
            rst.Update
            x = rst!余额
            rst.MoveNext
        Loop
        rst.Close
        Set rst = Nothing
    End If
End Sub