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我有一个名为 DUI_Database 的数据库。里面是一个名为 DUI_Cite_Numbers 的表。其中有两列-dui_cite_no 和状态。我有一些代码从两个文本框中获取一系列数字并将它们放在表格中。此外，状态列是 FREE 或 ALLOCATED。ALLOCATED 号码已使用且不能重复使用。免费就是可用的号码。

如果您查看表格，它看起来像这样，假设我输入了从 D100100 到 D100105 的范围

    DB Name: DUI_Database
    Table Name: DUI_Cite_Numbers

    dui_cite_no   status
    D100100       FREE
    D100101       FREE
    D100102       FREE
    D100103       FREE
    D100104       FREE
    D100105       FREE

然后一旦这些数字被使用，它看起来像这样-

    dui_cite_no   status
    D100100       ALLOCATED
    D100101       ALLOCATED
    D100102       ALLOCATED
    D100103       FREE
    D100104       FREE
    D100105       FREE

我需要有关 SELECT 语句的帮助。当表单打开时，我需要在该表单的文本框中使用下一个 FREE dui_cite_no。我不确定如何做到这一点。

我以为这行得通-

    "SELECT TOP(1) dui_cite_no FROM DUI_Cite_Numbers WHERE status = 'FREE'"

它不工作-看起来对吗？

因为一旦使用它，它就会在列中将其标记为 ALLOCATED -

    "UPDATE DUI_Cite_Numbers SET status = 'ALLOCATED' WHERE dui_cite_no = @nextcit"

我很困惑，可能是因为我已经开始这样做太久了。

这是整个代码块：

Public Function Get_Next_DUI_Cit_Number() As String
    ''Get the next available citation number from the database. If there is no free
    ''citation number then return a null string
    Dim nextcit As String = String.Empty
    Using DataConnection As New System.Data.SqlServerCe.SqlCeConnection("Data Source=C:\Program Files\DailyLog DUI\DUI_Database.sdf")
        DataConnection.Open()
        Dim SelectCommand As New System.Data.SqlServerCe.SqlCeCommand("SELECT TOP(1) dui_cite_no FROM DUI_Cite_Numbers WHERE status = 'FREE'", DataConnection)
        Dim DataReader As System.Data.SqlServerCe.SqlCeDataReader = SelectCommand.ExecuteReader()
        If DataReader.Read() Then
            nextcit = DataReader("dui_cite_no").ToString
            Using DataConnection2 As New System.Data.SqlServerCe.SqlCeConnection("Data Source=C:\Program Files\DailyLog DUI\DUI_Database.sdf")
                DataConnection2.Open()
                Dim UpdateCommand As New System.Data.SqlServerCe.SqlCeCommand("UPDATE DUI_Cite_Numbers SET status = 'ALLOCATED' WHERE dui_cite_no = @nextcit", DataConnection2)
                UpdateCommand.Parameters.AddWithValue("@nextcit", nextcit)
                UpdateCommand.ExecuteNonQuery()
                UpdateCommand.Dispose()
                DataConnection2.Close()
            End Using
        End If
        SelectCommand.Dispose()
        DataConnection.Close()
    End Using
End Function

然后当相应的表单打开时，代码如下：

Public Sub frmDUI_Citation_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) 处理 MyBase.Load

    Me.txt_dui_cite_no.Text = module1.Get_Next_DUI_Cit_Number() 'get the next available citation number and put it in the textbox

结束子

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1 回答 1

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如果要确保它们按顺序发出，则需要添加ORDER BY：

SELECT TOP(1) 
  dui_cite_no 
FROM 
  DUI_Cite_Numbers 
WHERE 
  status = 'FREE'
ORDER BY 
  dui_cite_Number
于 2012-09-15T18:32:08.840 回答