Ho do I calcualate duration of days between two date columns in DataGridView

• Use parameterized queries to fix the SQL Injection[^] vulnerability.
• Don't store connection and command objects as class-level fields; use local variables instead.
• Wrap objects that implement IDisposable in a Using block.

Private Function CreateConnection() As OleDbConnection
    Dim result As New OleDbConnection("** YOUR CONNECTION STRING HERE **")
    result.Open()
    Return result
End Function

Private Sub btnReport_SLA_Click(sender As Object, e As EventArgs) Handles btnReport_SLA.Click
    Try
        Using connection As OleDbConnection = CreateConnection()
            
            Using cmdDelete As OleDbCommand = connection.CreateCommand()
                cmdDelete.CommandText = "DELETE * FROM SLA"
                cmdDelete.ExecuteNonQuery()
            End Using
            
            Using cmdInsert As OleDbCommand = connection.CreateCommand()
                If DateAdded.Checked Then
                    cmdInsert.CommandText = "INSERT INTO SLA SELECT ID, Dial, ServiceName, SupplierName, DateAdded, SupplierFeedbackDate FROM Customers Where SupplierName = ? AND SupplierFeedbackDate <> '.... / .... / .....' AND DateAdded between ? And ?"
                    cmdInsert.Parameters.AddWithValue("SupplierName", cbo_Supplier.Text)
                    cmdInsert.Parameters.AddWithValue("DateAddedFrom", cbo_DateSent.Value)
                    cmdInsert.Parameters.AddWithValue("DateAddedTo", cbo_DateSentTo.Value)
                Else 
                    cmdInsert.CommandText = "INSERT INTO SLA SELECT ID, Dial, ServiceName, SupplierName, DateAdded, SupplierFeedbackDate FROM Customers Where SupplierName = ? AND SupplierFeedbackDate <> '.... / .... / .....'"
                    cmdInsert.Parameters.AddWithValue("SupplierName", cbo_Supplier.Text)
                End If
                
                cmdInsert.ExecuteNonQuery()
            End Using
            
            Using cmdUpdate As OleDbCommand = connection.CreateCommand()
                cmdUpdate.CommandText = "Update SLA SET ClosureDuration = SupplierFeedbackDate - DateAdded"
                cmdUpdate.ExecuteNonQuery()
            End Using
            
            If String.IsNullOrEmpty(cbo_Supplier.Text) Then
                MessageBox.Show("No supplier selected")
                Return
            End If
            
            Using cmdSelect As OleDbCommand = connection.CreateCommand()
                If DateAdded.Checked Then
                    cmdSelect.CommandText = "Select * From Customers Where SupplierName = ? AND DateAdded between ? And ? AND SupplierFeedbackDate <> '.... / .... / .....' "
                    cmdSelect.Parameters.AddWithValue("SupplierName", cbo_Supplier.Text)
                    cmdSelect.Parameters.AddWithValue("DateAddedFrom", cbo_DateSent.Value)
                    cmdSelect.Parameters.AddWithValue("DateAddedTo", cbo_DateSentTo.Value)
                Else
                    cmdSelect.CommandText = "Select * From Customers Where SupplierName = ? AND SupplierFeedbackDate <> '.... / .... / .....' "
                    cmdSelect.Parameters.AddWithValue("SupplierName", cbo_Supplier.Text)
                End If
                
                Using da As New OleDbDataAdapter(cmdSelect)
                    Dim ds As New DataSet()
                    da.Fill(ds, "Customers")
                    DataGridViewReports.DataSource = ds
                    DataGridViewReports.DataMember = "Customers"
                    lbl_RowCount.Text = DataGridViewReports.RowCount
                End Using
            End Using
            
            Using cmdAverage As OleDbCommand = connection.CreateCommand()
                cmdAverage.CommandText = "SELECT AVG(ClosureDuration) FROM SLA"
                Dim AVGResult As Integer = Convert.ToInt32(cmdAverage.ExecuteScalar())
                Dim SFound As String = If(AVGResult > 1, "s", "")
                
                If DateAdded.Checked Then
                    MessageBox.Show(String.Format("Response duration from {1} : {2} day{3}{0}{0}For all finished requests sent between {4} and {5}", _
                        vbNewLine, cbo_Supplier.Text, AVGResult, SFound, cbo_DateSent.Value, cbo_DateSentTo.Value));
                Else
                    MessageBox.Show(String.Format("Response duration from {1} : {2} day{3}{0}{0}For all finished requests.", _
                        vbNewLine, cbo_Supplier.Text, AVGResult, SFound));
                End If
            End Using
            
            Using cmdDelete As OleDbCommand = connection.CreateCommand()
                cmdDelete.CommandText = "DELETE * FROM SLA"
                cmdDelete.ExecuteNonQuery()
            End Using
            
        End Using
        
    Catch ex As OleDbException
        ' TODO: Log the full exception details somewhere:
        System.Diagnostics.Trace.TraceError(ex.ToString())
        MessageBox.Show(ex.Message)
    End Try
End Sub