Question

我正在Windows窗体（vb.net）中创建一个新应用程序，该应用程序会根据某些条件每天检查是否有任何记录添加到SQL数据库中。问题是我无法弄清楚，如果查询返回空值，如何在标签框中显示一些消息。

Public Sub LoadUser2()
    Dim dt As New DataTable()
    Dim Con = New SqlConnection("Server=my server;database=My db;Integrated Security = True")
    Con.Open()
    Dim cmd As SqlCommand = New SqlCommand("select sum(Minutes)/60 as 'Ore Logate' from [dbo].[UserActivity]
                                            where team = 'Xaver' and username like '%alnecula%' and Convert(date,DateCreated) = Convert(date,getdate())", Con)
    Dim da As SqlDataAdapter = New SqlDataAdapter(cmd)
    da.SelectCommand = cmd
    da.Fill(dt)

    If dt.Rows.Count > 0 Then
        Txtgvlad.Text = "Logged Hours: " & dt.Rows(0)("Username").ToString
    Else
        Txtgvlad.Text = "You haven't logged any hours"
    End If
    Con.Close()
End Sub

因此，我想显示记录的小时数（如果有的话），否则显示字符串消息，通知用户没有记录的小时数。当前，如果结果为空，代码将显示Logged Hours:。

Answer 1

您的实际代码存在的问题是，即使where条件不返回任何记录，您在表中仍然具有包含NULL的行。因此，您的测试始终是正确的。您只需在Sql Server Management Studio中复制查询并运行它即可进行测试。

相反，您可以使用ExecuteScalar来获取结果

Dim cmd As SqlCommand = New SqlCommand("select sum(Minutes)/60 as 'Ore Logate' 
                                        from [dbo].[UserActivity]
                                        where team = 'Xaver'  
                                          and username like '%alnecula%' 
                                          and Convert(date,DateCreated) = Convert(date,getdate())", Con)
Dim result = cmd.ExecuteScalar()
If Not IsDBNull(result) Then 
    Txtgvlad.Text = "Logged Hours: " & result.ToString
Else
    Txtgvlad.Text = "You haven't logged any hours"
End If

还要考虑到该查询可以返回零（但不能为NULL），并且在这种情况下IF条件为true。如果即使在返回零的情况下也要编写错误消息，则需要在If中添加一个AndAlso条件

If Not IsDBNull(result) AndAlso result.ToString <> "0" Then

Answer 2

解决这个问题的一种好方法是使用sqldatareader。因此，基本上，您通常会像平常一样执行Sp，但是使用阅读器执行它，然后在if语句中使用.hasrows。例如您提供的内容

Public Sub LoadUser2()

    Dim Con = New SqlConnection("Server=my server;database=My db;Integrated Security = True")
    Con.Open()
    Dim cmd As new SqlCommand = New SqlCommand("select sum(Minutes)/60 as 'Ore Logate' from [dbo].[UserActivity]
                                        where team = 'Xaver' and username like '%alnecula%' and Convert(date,DateCreated) = Convert(date,getdate())", Con)


   using RDR = cmd.executeReader
    if RDR.has rows
       IF RDR("YourColumnName").tostring() = "" then
          Txtgvlad.Text = "You haven't logged any hours"
       else
        Txtgvlad.Text = "Logged Hours: " & RDR("Username").ToString

    else
        Txtgvlad.Text = "You haven't logged any hours"
    end if
  end using


    Con.Close()
End Sub

您的原始行可能仍在返回而有行，可能是因为它返回的是一行，而只是空行。

如何检查查询是否返回空值并将其添加到标签文本？

2 个答案: