vb.net IsDBNull如果语句未显示msgbox

Question

我试图检查访问数据库中的空记录，并认为我正在使用的代码可以工作。 应该发生的是，如果数据库中没有该表的记录，则显示msgbox。 但是，当我运行代码时，什么都没有显示。 我是正确使用IsDBNull还是有更好的方法来做到这一点？ 我开始使用params代替＆引用，并且在测试后应对此进行更改。 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。 该方法的文档指出：

返回一个布尔值，该布尔值指示表达式是否对System.DBNull类求值。

您发送到IsDBNull方法中的sql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IsDBNull SQL后，可以使用IsDBNull来检查结果中的特定字段是否为NULL。 要执行所需的操作，只需在调用Fill之后检查数据表中是否包含任何行。

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

Answer 2

IsDbNull将不会调用您的数据库。 它只是测试参数是否等于DbNull.Value，如果您使用字符串调用它，则永远不会如此。

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