Question

我正在尝试检查访问数据库中的空记录，并认为我使用的代码可以正常工作。应该发生的是，如果db中没有该表的记录，则显示msgbox。但是，运行代码时没有显示任何内容。我使用IsDBNull是正确的方法还是有更好的方法来做到这一点。我正在努力使用params代替＆amp;参考，这将在测试后更改。非常感谢。

Dim con1 As New OleDbConnection("Provider = Microsoft.ACE.OLEDB.12.0;Data Source=C:\domain\test.accdb")    
Dim sql As String
            sql = "SELECT * FROM Departments where Customer = '" & customer & "'"
            If IsDBNull(sql) Then
                MessageBox.Show("No record") <---THIS NOT FIRING
                ' This is our DataAdapter. This executes our SQL Statement above against the Database
                ' we defined in the Connection String
            Else
                Dim adapter As New OleDbDataAdapter(sql, con1)
                ' Gets the records from the table and fills our adapter with those.
                Dim dt As New DataTable("Departments")
                adapter.Fill(dt)
                ' Assigns our DataSource on the DataGridView
                dgv1.DataSource = dt
                '
                Dim sql1 As String
                sql1 = "SELECT * FROM Departments"
                Dim adapter1 As New OleDbDataAdapter(sql1, con1)
                Dim cmd1 As New OleDbCommand(sql1, con1)
                'Dim dt1 As New DataTable("Departments")
                con1.Open()
                Dim myreader As OleDbDataReader = cmd1.ExecuteReader
                myreader.Read()

                con1.Close()
            End If

Answer 1

是的，您使用IsDBNull完全错误。该方法的文档说明：

返回一个布尔值，指示表达式是否计算为System.DBNull类。

您发送到sql方法的IsDBNull变量显然不会评估为System.DBNull，因为您自己将其设置为其他值。

执行SQL后使用IsDBNull，检查结果中的特定字段是否为NULL。要做你想做的事，你只需要在调用Fill后检查数据表中是否有任何行。

Dim dt As New DataTable("Departments")
adapter.Fill(dt)
If dt.Rows.Count = 0 Then MessageBox.Show("No record")

Answer 2

IsDbNull不会调用您的数据库。它只是测试参数是否等于DbNull.Value，如果用字符串调用它将永远不会出现这种情况。

你可以试试这个

Dim con1 As New OleDbConnection("Provider = Microsoft.ACE.OLEDB.12.0;Data Source=C:\domain\test.accdb")
Dim sql As String
sql = "SELECT COUNT(*) FROM Departments where Customer = '" & customer & "'"
con1.Open()
Dim cmd1 As New OleDbCommand(sql, con1)
If cmd1.ExecuteScalar() = 0 Then
  MessageBox.Show("No record")
Else
  Dim adapter As New OleDbDataAdapter(sql, con1)
  ' Gets the records from the table and fills our adapter with those.
  Dim dt As New DataTable("Departments")
  adapter.Fill(dt)
  ' Assigns our DataSource on the DataGridView
  dgv1.DataSource = dt
  '
  Dim sql1 As String
  sql1 = "SELECT * FROM Departments"
  Dim adapter1 As New OleDbDataAdapter(sql1, con1)
  cmd1 = New OleDbCommand(sql1, con1)
  'Dim dt1 As New DataTable("Departments")
  Dim myreader As OleDbDataReader = cmd1.ExecuteReader
  myreader.Read()

  con1.Close()
End If

vb.net IsDBNull如果语句没有显示msgbox

2 个答案: