Question

美好的一天这是我之前问题的延续，因为它已经解决了。

Issue with Custom Date Format in VB.NET

现在的问题是我在phpMyAdmin中测试我的SQL查询时返回了一个值

这意味着rd.GetString(0)的值为＆＃34; 3＆＃34;

但是当vb.net加载该值时，它将返回错误：

Additional information: Data is Null. This method or property cannot be called on Null values.

为什么Data是NULL但我的SQL截图工作正常？

这是引用

的错误

If Convert.ToDouble(rd.GetString(0)) >= 80.0 Then

或者我用这个

If  rd.GetDecimal(0) >= 80.0 Then

相同的结果

我的整个代码：

    Dim startDayMonth As DateTime = FirstDayOfMonth(Now).AddHours(6)
    Dim lastDayMonth As DateTime = LastDayOfMonth(Now).AddHours(22)

    Dim qry As String = "SELECT SUM(totalWorkHour) as totalWorkHour FROM tbltimesheet WHERE studInfoID=@id " _
        & "AND timeIn BETWEEN @checkAM AND @checkPM"

    Using con As New MySqlConnection("server=192.168.*.***; user=dummyUsername; password=dummyPasswrd; database=dummyDBName;"), _
        cmd As New MySqlCommand(qry, con)

        cmd.Parameters.Add("@id", MySqlDbType.Int32).Value = Convert.ToInt32(Me.txtStudID.Text)
        cmd.Parameters.Add("@checkAM", MySqlDbType.DateTime).Value = startDayMonth
        cmd.Parameters.Add("@checkPM", MySqlDbType.DateTime).Value = lastDayMonth
        con.Open()

        Using rd As MySqlDataReader = cmd.ExecuteReader()
            'Better practice here would be to return the value (if any) from this function
            If rd.Read() Then
                If rd.GetDecimal(0) >= 80.0 Then
                    MsgBox("Hurray! You already reach your target: " & rd.GetString(0), MsgBoxStyle.Information, Title:="Result")
                Else
                    MsgBox("Your Total Work Hour: " & rd.GetString(0), MsgBoxStyle.Information, Title:="Result")
                End If
            Else
                MsgBox("No Record Found", MsgBoxStyle.Critical, Title:="Error")
            End If
        End Using
    End Using

    rd.Close()
    con.Close()

Answer 1

这里有很多小修正：

'Just leave these as a datetime
Dim StartDayMonth As DateTime = FirstDayOfMonth(Now).AddHours(6)
Dim LastDayMonth As DateTime = LastDayOfMonth(Now).AddHours(22)

Dim qry As String = _
    "SELECT SUM(totalWorkHour) as totalWorkHour " & _ 
    " FROM tbltimesheet " & _
    " WHERE studInfoID=@id AND timeIn BETWEEN @checkAM AND @checkPM"

'.Net uses Connection Pooling for database objects...
'    This means create a new connection for every query in most cases
'    but limit the scope and duration of the connection as much as possible
'    (wait until the last possible moment to call .Open())
Using con As New MySqlConnection("connection string here"), _
      cmd As New MySqlCommand(qry, con)

    cmd.Parameters.Add("@id", MySqlDbType.Int32).Value =  Convert.ToInt32(Me.txtStudID.Text)
    cmd.Parameters.Add("@checkAM", MySqlDbType.DateTime).Value = StartDayMonth
    cmd.Parameters.Add("@checkPM", MySqlDbType.DateTime).Value = LastDayMonth
    con.Open()

    Using rd As MySqlDataReader = cmd.ExecuteReader()
        'Better practice here would be to return the value (if any) from this function
        If rd.Read()
            Dim hours As Double = rd.GetDouble(0)
            If hours >= 80.0 Then
                MsgBox("Hurray! You already reach your target: " & hours.ToString(), MsgBoxStyle.Information, Title:="Result")
            Else
                MsgBox("Your Total Work Hour: " & hours.ToString(), MsgBoxStyle.Information, Title:="Result")
            End If
        Else 
            MsgBox("No Record Found", MsgBoxStyle.Critical, Title:="Error")
        End If
    End Using
End Using

现在我也可以看到模块了，你想要更像这样的代码：

Module db
    'Note that this is PRIVATE
    '  Goal is to make sure any db access in your code really
    '  does use the interface provided by your db module.
    Private ConnectionString As String  = "server=192.168.*.***; user=******; password=*****; database=dbsalog;"

    'Note the use of types here in the function definition.
    'Also note how I separate concerns here:
    '   Limit functions in this Module to just database access doing database things
    '   Let your presentation code in your form worry about MsgBox() and other concerns
    Public Function GetWorkHours(StudentId As Integer) As Double
        Dim StartDayMonth As DateTime = FirstDayOfMonth(Now).AddHours(6)
        Dim LastDayMonth As DateTime = LastDayOfMonth(Now).AddHours(22)
        Dim qry As String = _
            "SELECT SUM(totalWorkHour) as totalWorkHour " & _ 
            " FROM tbltimesheet " & _
            " WHERE studInfoID=@id AND timeIn BETWEEN @checkAM AND @checkPM"

        Using con As New MySqlConnection(ConnectionString), _
              cmd As New MySqlCommand(qry, con)

            cmd.Parameters.Add("@id", MySqlDbType.Int32).Value = StudentId
            cmd.Parameters.Add("@checkAM", MySqlDbType.DateTime).Value = StartDayMonth
            cmd.Parameters.Add("@checkPM", MySqlDbType.DateTime).Value = LastDayMonth
            con.Open()

            Using rd As MySqlDataReader = cmd.ExecuteReader()
                If rd.Read() Return rd.GetDouble(0) 
            End Using
        End Using
        Return Nothing
    End Function
End Module

您可以从程序的表单或其他区域调用，如下所示：

Dim workHours As Double = db.GetWorkHours(Convert.ToInt32(Me.txtStudId.Text))
If workHours = Nothing Then
    MsgBox("No Record Found", MsgBoxStyle.Critical, Title:="Error")
ElseIf workHours >= 80.0 Then
    MsgBox("Hurray! You already reach your target: " & workHours.ToString(), MsgBoxStyle.Information, Title:="Result")
Else
    MsgBox("Your Total Work Hour: " & workHours.ToString(), MsgBoxStyle.Information, Title:="Result")
End If

即使我在VB.NET中的SQL上有值，数据返回也是NULL

1 个答案: