Question

我正在尝试从ACCESS数据库中提取数据。

实际上，代码有效，并且没有错误...... 但是，如果记录不存在，我似乎无法显示消息框。它只返回一个空字符串。

        Using dbCon = New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;" & "Data Source = '" & Application.StartupPath & "\Res\T500G.accdb'")

        dbCon.Open()

        Dim Query1 As String = "SELECT SUM(Total) FROM [T500] WHERE Pro=@Pro"

        Dim cmd1 As OleDb.OleDbCommand = New OleDbCommand(Query1, dbCon)
        cmd1.Parameters.AddWithValue("@Pro", ComboBoxBP.SelectedItem.ToString)

        Dim reader As OleDb.OleDbDataReader
        reader = cmd1.ExecuteReader


        While reader.Read()

            TextBox1.Text = reader.GetValue(0).ToString

        End While

        reader.Close()
        dbCon.Close()

    End Using

我尝试在文本框中使用If reader.hasrows then显示结果，否则显示消息框等，但它不起作用。

        If reader.HasRows Then
            While reader.Read()

                TextBox1.Text = reader.GetValue(0).ToString

            End While
        Else

            MessageBox.Show("asd")

        End If

如果我从.ToString删除了reader.GetValue(0)，如果数据库中不存在组合框中的所选项目，则会收到错误消息。无法将DBNull转换为整数或其他内容。

所以我的问题是，如果记录“@Pro”不存在，如何显示消息框？

感谢〜

使用此

修复（解决方法）

Dim ValueReturned As String
        While reader.Read()

            ValueReturned = reader.GetValue(0).ToString

            If ValueReturned = "" Then

                MessageBox.Show("Not Found", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)

            Else

                MetroTextBox1.Text = ValueReturned

            End If

        End While

Answer 1

您可以将查询更改为（这适用于SQL-Server）：

IF EXISTS (SELECT * FROM [T500] WHERE Pro = @Pro) SELECT SUM(Total) FROM [T500] WHERE Pro = @Pro ELSE SELECT 'Not Found'

并将您的代码更改为：

    Dim ValueReturned As String
    While reader.Read()

        ValueReturned = reader.GetValue(0).ToString

    End While
    If ValueReturned Is Nothing OrElse ValueReturned = "Not Found" Then
        MessageBox.Show("Not Found", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
    Else
        TextBox1.Text = ValueReturned
    End If

Answer 2

使用OleDbDataReader对于您的查询来说不是最理想的，因为它永远不会返回一组要遍历的记录：

Dim sql = "SELECT SUM(...=@p1"

' rather than sprinkling you connection string all over
' the app, you can make a function returning one
Using conn As OleDbConnection = GetConnection(),
        cmd As New OleDbCommand(sql, GetConnection())
    conn.Open())

    ' ToDo: Check that ComboBoxBP.SelectedItems.Count >0 before this
    cmd.Parameters.AddWithValue("@p1", ComboBoxBP.SelectedItem.ToString)

    ' execute the query, get a result
    Dim total = cmd.ExecuteScalar

    ' if there is no matches, OleDb returns DBNull
    If total Is System.DBNull.Value Then
        ' no matches
        MessageBox.Show("No matching records!"...)
    Else
        MessageBox.Show("The Total is: " & total.ToString()...)
    End If

End Using   ' disposes of the Connection and Command objects

另外，您可以使用If IsDBNull(total) Then...。如果你愿意，你也可以转换它：

Dim total = cmd.ExecuteScalar.ToString()
' DBNull will convert to an empty string
If String.IsNullOrEmpty(total) Then
    MessageBox.Show("No Soup For You!")
Else
    ...
End If

检测OleDB COUNT查询是否存在数据

2 个答案: