有关DBNULL值的帮助 [英] Help on DBNULL values

问题描述

您好，请问我正在一个项目上.在一个网页中，有一个文本框可以选择用户必须给定的两个给定期间之间的总金额.但是，如果没有给定的日期，它将引发异常，指出无法将对象从DBNull强制转换为其他类型".不知道该怎么做才能使它正常工作.我仍然是vb和sql的婴儿.任何帮助都受到高度赞赏.以下是我的代码

Hello,please i am working on a project.In one of the web page there is a textbox to select the total amount between two given periods which the user must give.The code works fine if the given dates are present in the database.But if those date given are not present it throws an exception saying "Object cannot be cast from DBNull to other types". Don''t know what to do to make it work.i am still a baby in vb and sql.Any help is highly appreciated.Below is my code

Protected Sub SearchButton_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles SearchButton.Click



        If TxtEndDate.Text = String.Empty Then
        ElseIf TxtStartDate.Text = String.Empty Then
            Exit Sub
        Else

            Try


                Dim connectionstring As String = ConfigurationManager.ConnectionStrings("LordsBannerMotorsConnectionString1").ConnectionString
                Dim insertsql As String = "SELECT SUM(Amount) As [Total Amount] FROM MoneyPaidToBank WHERE " &
                                         " Bank_Location=@Location AND Date BETWEEN @Start AND @End"
                Using connection As New SqlConnection(connectionstring)
                    connection.Open()

                    Dim mycommand As New SqlCommand(insertsql, connection)
                    mycommand.Parameters.Add("@Location", SqlDbType.VarChar, 50)
                    mycommand.Parameters("@Location").Value = TotalMoneyDropDownList1.SelectedValue
                    mycommand.Parameters.Add("@Start", SqlDbType.SmallDateTime)
                    mycommand.Parameters("@Start").Value = CType(TxtStartDate.Text, Date)
                    mycommand.Parameters.Add("@End", SqlDbType.SmallDateTime)
                    mycommand.Parameters("@End").Value = CType(TxtEndDate.Text, Date)
                    Dim str As String = ""
                    Dim reader As SqlDataReader = mycommand.ExecuteReader
                    If reader.HasRows Then
                        While reader.Read
                            str &= Convert.ToDouble(reader.Item("Total Amount"))

                        End While
                    Else
                        SearchLabel.Text = "Record not found"
                    End If

                    SearchLabel.Text = "The total Amount is " & str

                    connection.Close()
                End Using
            Catch ex As Exception
                SearchLabel.Text = ex.Message

            End Try
        End If

推荐答案

如果未找到指定位置和日期范围的金额，则SUM函数将返回NULL.
使用 COLEASCE() [ ISNULL() [ ^ ]函数，如下所示:

If the Amount was not found for the specified location and date range, SUM function returns NULL.
Use COLEASCE()[^] or ISNULL()[^] function, like this:

SELECT SUM(ISNULL(Amount,0)) As [Total Amount]
FROM MoneyPaidToBank
WHERE Bank_Location=@Location AND Date BETWEEN @Start AND @End

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