[vb.net] delete from

[x69asterix]

non riesco ad eliminare tutti i record dove il campo è vuoto.
io eseguo:

Dim a As String = String.Empty Or "NULL"

            dataAdapter = New SqlClient.SqlDataAdapter("DELETE FROM Storico WHERE Note = '" & a & "'", conn)
            commandBuilder = New SqlClient.SqlCommandBuilder(dataAdapter)
            dataSet = New Data.DataSet
            dataAdapter.Fill(dataSet)

            DataGridView1.DataSource = Nothing
            dataAdapter = New SqlClient.SqlDataAdapter("SELECT * FROM Storico WHERE Reparto = '" & ComboBox1.Text & "' ORDER BY Cognome ASC", conn)
            commandBuilder = New SqlClient.SqlCommandBuilder(dataAdapter)
            dataSet = New Data.DataSet
            dataAdapter.Fill(dataSet)
            DataGridView1.DataSource = dataSet.Tables(0)

dove sbaglio?