RadGrid for ASP.NET AJAX

public partial class _Default : System.Web.UI.Page
{
    //Declare a global DataTable dtTable       
    public static DataTable dtTable;
    //Get the connectionstring from the webconfig and declare a global SqlConnection "SqlConnection"   
    public static string connectionString = ConfigurationManager.AppSettings["ConnectionString"];
    public SqlConnection SqlConnection = new SqlConnection(connectionString);
    //Declare a global SqlDataAdapter SqlDataAdapter     
    public SqlDataAdapter SqlDataAdapter = new SqlDataAdapter();
    //Declare a global SqlCommand SqlCommand      
    public SqlCommand SqlCommand = new SqlCommand();
    protected void RadGrid1_NeedDataSource(object source, Telerik.Web.UI.GridNeedDataSourceEventArgs e)
    {
        //Populate the Radgrid      
        dtTable = new DataTable();
        //Open the SqlConnection     
        SqlConnection.Open();
        try
        {
            //Select Query to populate the RadGrid with data from table Employees.     
            string selectQuery = "SELECT * FROM Employees";
            SqlDataAdapter.SelectCommand = new SqlCommand(selectQuery, SqlConnection);
            SqlDataAdapter.Fill(dtTable);
            RadGrid1.DataSource = dtTable;
        }
        finally
        {
            //Close the SqlConnection       
            SqlConnection.Close();
        }
    }
    protected void RadGrid1_DeleteCommand(object source, Telerik.Web.UI.GridCommandEventArgs e)
    {
        //Get the GridDataItem of the RadGrid        
        GridDataItem item = (GridDataItem)e.Item;
        //Get the primary key value using the DataKeyValue.       
        string EmployeeID = item.OwnerTableView.DataKeyValues[item.ItemIndex]["EmployeeID"].ToString();
        try
        {
            //Open the SqlConnection                 SqlConnection.Open();               
            string deleteQuery = "DELETE from Employees where EmployeeID='" + EmployeeID + "'";
            SqlCommand.CommandText = deleteQuery;
            SqlCommand.Connection = SqlConnection;
            SqlCommand.ExecuteNonQuery();
            //Close the SqlConnection     
            SqlConnection.Close();
        }
        catch (Exception ex)
        {
            RadGrid1.Controls.Add(new LiteralControl("Unable to delete Employee. Reason: " + ex.Message));
            e.Canceled = true;
        }
    }
    protected void RadGrid1_UpdateCommand(object source, Telerik.Web.UI.GridCommandEventArgs e)
    {
        //Get the GridEditableItem of the RadGrid      
        GridEditableItem editedItem = e.Item as GridEditableItem;
        //Get the primary key value using the DataKeyValue.      
        string EmployeeID = editedItem.OwnerTableView.DataKeyValues[editedItem.ItemIndex]["EmployeeID"].ToString();
        //Access the textbox from the edit form template and store the values in string variables.   
        string LastName = (editedItem["LastName"].Controls[0] as TextBox).Text;
        string FirstName = (editedItem["FirstName"].Controls[0] as TextBox).Text;
        string Title = (editedItem["Title"].Controls[0] as TextBox).Text;
        string Address = (editedItem["Address"].Controls[0] as TextBox).Text;
        string City = (editedItem["City"].Controls[0] as TextBox).Text;
        try
        {
            //Open the SqlConnection       
            SqlConnection.Open();
            //Update Query to update the Datatable  
            string updateQuery = "UPDATE Employees set LastName='" + LastName + "',FirstName='" + FirstName + "',Title='" + Title + "',Address='" + Address + "',City='" + City + "' where EmployeeID='" + EmployeeID + "'";
            SqlCommand.CommandText = updateQuery;
            SqlCommand.Connection = SqlConnection;
            SqlCommand.ExecuteNonQuery();
            //Close the SqlConnection      
            SqlConnection.Close();
        }
        catch (Exception ex)
        {
            RadGrid1.Controls.Add(new LiteralControl("Unable to update Employee. Reason: " + ex.Message));
            e.Canceled = true;
        }
    }
    protected void RadGrid1_InsertCommand(object source, Telerik.Web.UI.GridCommandEventArgs e)
    {
        //Get the GridEditFormInsertItem of the RadGrid             
        GridEditFormInsertItem insertedItem = (GridEditFormInsertItem)e.Item;
        //string EmployeeID = (insertedItem["EmployeeID"].Controls[0] as TextBox).Text;             
        string LastName = (insertedItem["LastName"].Controls[0] as TextBox).Text;
        string FirstName = (insertedItem["FirstName"].Controls[0] as TextBox).Text;
        string Title = (insertedItem["Title"].Controls[0] as TextBox).Text;
        string Address = (insertedItem["Address"].Controls[0] as TextBox).Text;
        string City = (insertedItem["City"].Controls[0] as TextBox).Text;
        try
        {
            //Open the SqlConnection               
            SqlConnection.Open();
            //Update Query to insert into  the database               
            string insertQuery = "INSERT into  Employees(LastName,FirstName,Title,Address,City) values('" + LastName + "','" + FirstName + "','" + Title + "','" + Address + "','" + City + "')";
            SqlCommand.CommandText = insertQuery;
            SqlCommand.Connection = SqlConnection;
            SqlCommand.ExecuteNonQuery();
            //Close the SqlConnection                
            SqlConnection.Close();
        }
        catch (Exception ex)
        {
            RadGrid1.Controls.Add(new LiteralControl("Unable to insert Employee. Reason: " + ex.Message));
            e.Canceled = true;
        }
    }
}