vb.net IsDBNull如果语句未显示msgbox

Question

I am trying to check for empty records in an access db and thought that the code I am using would work. What is supposed to happen, is if there are no records for that table in the db, then the msgbox is displayed. However, nothing is displaying when I run the code. Am I using IsDBNull the correct way or is there a better way to do it. I am getting to grips with using params instead of & references and this shall be changed after testing. Many thanks.

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

Yes, you are using IsDBNull completely wrong. The documentation for that method states:

Returns a Boolean value that indicates whether an expression evaluates to the System.DBNull class.

The sql variable you're sending into the IsDBNull method clearly does not evaluate to System.DBNull , as you just set it to some other value yourself.

You use IsDBNull after you've executed your SQL, to check to see if a specific field in your result was NULL. To do what you want, you'll just want to check if the data table has any rows it in after you call Fill .

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

Answer 2

IsDbNull will not call your database. It just tests if the parameter is equal to DbNull.Value which will never be the case if you call it with a string.

you could try this

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