Query Doctypes with no links

Hello i have a query in my pre_delivery_inspection doctype where i want to get the Delivery Note for the item inspected in the pre delivery by filtering the sales invoice, item_code and serial_no.
Any help will vbe much appreciated, check code below.

class PreDeliveryInspection(Document):
def onload(self):
“”" Load delivery Note link “”"
dashboard_links_html = self.load_delivery_note()
self.set_onload(“dashboard_links_html”, {“display”: dashboard_links_html})

def load_delivery_note(self):
	delivery_note_links = []
	delivery_note_links = frappe.db.sql("""
		SELECT
			tdni.item_code,
			tdni.parent,
			tdni.serial_no,
			tdni.against_sales_invoice
		FROM (`tabDelivery Note Item` tdni)
		LEFT JOIN
			(`tabPre Delivery Inspection` tpdn)
		ON  tdni.against_sales_invoice = tpdn.reference_name
		where 
			tdni.against_sales_invoice = %(cust)s
		AND
			tdni.item_code = %(item)s
		AND
			tdni.serial_no = %(serial)s		
		""",{'cust': self.reference_name, 'item':self.item_code, 'serial':self.item_serial_no}, as_dict=True)
	self.set_onload("delivery_note_links", delivery_note_links)
	if delivery_note_links:
		frappe.throw("delivery_note_links not empty")
	return frappe.render_template("templates/pre_delivery_links.html", {"delivery_note_links": delivery_note_links})