Sql “选择不存在/不存在的位置”子查询（VB.NET，Access）出现问题

我有个问题需要帮助。 代码如下：

Private Sub dtpStartDate_ValueChanged(sender As Object, e As EventArgs) Handles dtpStartDate.ValueChanged
    startDateChanged = 1

    If endDateChanged = 1 Then
        cbLocation.Enabled = True
        cbLocation.Items.Clear()
        cbLocation.Items.Add(New ListViewItem(""))
        Dim unbookedLocationsSQL As String = "SELECT locationID FROM Locations WHERE NOT EXISTS (Select LocationID FROM Bookings WHERE @startDate <= bookingEndDate AND bookingStartDate <= @endDate)"
        Dim unbookedLocationsCommand = New OleDbCommand(unbookedLocationsSQL, globalVariables.objConnection)
        Dim unbookedLocationsAdapter As New OleDbDataAdapter(unbookedLocationsSQL, globalVariables.objConnection)
        Dim unbookedLocationsDataSet As New DataSet

        unbookedLocationsCommand.Parameters.AddWithValue("startDate", dtpStartDate.Value)
        unbookedLocationsCommand.Parameters.AddWithValue("endDate", dtpEndDate.Value)

        unbookedLocationsAdapter.Fill(unbookedLocationsDataSet, "Locations")

        With cbLocation
            .DataSource = unbookedLocationsDataSet.Tables("Locations")
            .DisplayMember = "locationID"
            .ValueMember = "locationID"
        End With
    End If
End Sub

首先，如果将sql语句更改为从位置选择*，则组合框将显示所有位置

我想达到的是这一点；当有人更改两个日期时间选择器时，组合框将启用并填充在这两个日期之间未预订的位置列表，该列表由“预订”表确定。 我知道SQL语句是错误的。我尝试了各种各样的组合，并尝试了分离一些零碎的东西，但我无法得到任何子查询来实现我想要的

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任何帮助都将不胜感激。

我觉得这里有点不对劲

WHERE @startDate <= bookingEndDate AND bookingStartDate <= @endDate

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如果我理解正确的话，Bookings表中的每个条目都反映了一次预订，即使是在特定地点。在这种情况下，您需要：

WHERE @startDate >= bookingEndDate OR bookingStartDate >= @endDate

而不是

WHERE @startDate <= bookingEndDate AND bookingStartDate <= @endDate

此外，如果Bookings表可以为同一位置包含多个条目，则这将不起作用：您必须确保该位置的所有预订都与输入日期不重叠

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由于我还不能对其他答案发表评论，我将在这里注意到Codemunkee的答案是错误的-使用他的查询，其中bookingStartDate>=@startDate和bookingEndDate这里是代码，它现在正在工作：

 Private Sub dtpStartDate_ValueChanged(sender As Object, e As EventArgs) Handles dtpStartDate.ValueChanged
        startDateChanged = 1

        If endDateChanged = 1 Then
            cbLocation.Enabled = True
            Me.Refresh()
            cbLocation.Items.Add(New ListViewItem(""))
            Dim unbookedLocationsSQL As String = "SELECT * FROM Locations WHERE LocationID NOT IN (SELECT LocationID FROM Bookings WHERE bookingEndDate >= @startDate AND bookingStartDate <= @endDate)"
            Dim unbookedLocationsCommand = New OleDbCommand(unbookedLocationsSQL, globalVariables.objConnection)
            Dim unbookedLocationsAdapter As New OleDbDataAdapter(unbookedLocationsSQL, globalVariables.objConnection)
            Dim unbookedLocationsDataSet As New DataSet

            unbookedLocationsCommand.Parameters.AddWithValue("startDate", dtpStartDate.Value)
            unbookedLocationsCommand.Parameters.AddWithValue("endDate", dtpEndDate.Value)
            unbookedLocationsAdapter.SelectCommand = unbookedLocationsCommand
            unbookedLocationsAdapter.Fill(unbookedLocationsDataSet, "Locations")

            With cbLocation
                .DataSource = unbookedLocationsDataSet.Tables("Locations")
                .DisplayMember = "LocationName"
                .ValueMember = "LocationID"
            End With
        End If
    End Sub

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您是否尝试过隔离此查询？从@startDate所在的预订中选择LocationID

WHERE @startDate <= bookingEndDate AND bookingStartDate <= @endDate

 Private Sub dtpStartDate_ValueChanged(sender As Object, e As EventArgs) Handles dtpStartDate.ValueChanged
        startDateChanged = 1

        If endDateChanged = 1 Then
            cbLocation.Enabled = True
            Me.Refresh()
            cbLocation.Items.Add(New ListViewItem(""))
            Dim unbookedLocationsSQL As String = "SELECT * FROM Locations WHERE LocationID NOT IN (SELECT LocationID FROM Bookings WHERE bookingEndDate >= @startDate AND bookingStartDate <= @endDate)"
            Dim unbookedLocationsCommand = New OleDbCommand(unbookedLocationsSQL, globalVariables.objConnection)
            Dim unbookedLocationsAdapter As New OleDbDataAdapter(unbookedLocationsSQL, globalVariables.objConnection)
            Dim unbookedLocationsDataSet As New DataSet

            unbookedLocationsCommand.Parameters.AddWithValue("startDate", dtpStartDate.Value)
            unbookedLocationsCommand.Parameters.AddWithValue("endDate", dtpEndDate.Value)
            unbookedLocationsAdapter.SelectCommand = unbookedLocationsCommand
            unbookedLocationsAdapter.Fill(unbookedLocationsDataSet, "Locations")

            With cbLocation
                .DataSource = unbookedLocationsDataSet.Tables("Locations")
                .DisplayMember = "LocationName"
                .ValueMember = "LocationID"
            End With
        End If
    End Sub