### Instruction:

Convert text to sql: What is the product ID of the most frequently ordered item on invoices? | ref_payment_methods : payment_method_code , payment_method_description | ref_service_types : service_type_code , parent_service_type_code , service_type_description | addresses : address_id , line_1 , line_2 , city_town , state_county , other_details | products : product_id , product_name , product_price , product_description , other_product_service_details | marketing_regions : marketing_region_code , marketing_region_name , marketing_region_descriptrion , other_details | clients : client_id , address_id , customer_email_address , customer_name , customer_phone , other_details | drama_workshop_groups : workshop_group_id , address_id , currency_code , marketing_region_code , store_name , store_phone , store_email_address , other_details | performers : performer_id , address_id , customer_name , customer_phone , customer_email_address , other_details | customers : customer_id , address_id , customer_name , customer_phone , customer_email_address , other_details | stores : store_id , address_id , marketing_region_code , store_name , store_phone , store_email_address , other_details | bookings : booking_id , customer_id , workshop_group_id , status_code , store_id , order_date , planned_delivery_date , actual_delivery_date , other_order_details | performers_in_bookings : order_id , performer_id | customer_orders : order_id , customer_id , store_id , order_date , planned_delivery_date , actual_delivery_date , other_order_details | order_items : order_item_id , order_id , product_id , order_quantity , other_item_details | invoices : invoice_id , order_id , payment_method_code , product_id , order_quantity , other_item_details , order_item_id | services : service_id , service_type_code , workshop_group_id , product_description , product_name , product_price , other_product_service_details | bookings_services : order_id , product_id | invoice_items : invoice_item_id , invoice_id , order_id , order_item_id , product_id , order_quantity , other_item_details | clients.address_id = addresses.address_id | drama_workshop_groups.address_id = addresses.address_id | performers.address_id = addresses.address_id | customers.address_id = addresses.address_id | stores.marketing_region_code = marketing_regions.marketing_region_code | stores.address_id = addresses.address_id | bookings.workshop_group_id = drama_workshop_groups.workshop_group_id | bookings.customer_id = clients.client_id | performers_in_bookings.order_id = bookings.booking_id | performers_in_bookings.performer_id = performers.performer_id | customer_orders.store_id = stores.store_id | customer_orders.customer_id = customers.customer_id | order_items.product_id = products.product_id | order_items.order_id = customer_orders.order_id | invoices.payment_method_code = ref_payment_methods.payment_method_code | invoices.order_id = bookings.booking_id | invoices.order_id = customer_orders.order_id | services.service_type_code = ref_service_types.service_type_code | services.workshop_group_id = drama_workshop_groups.workshop_group_id | bookings_services.product_id = services.service_id | bookings_services.order_id = bookings.booking_id | invoice_items.order_id = bookings_services.order_id | invoice_items.product_id = bookings_services.product_id | invoice_items.invoice_id = invoices.invoice_id | invoice_items.order_item_id = order_items.order_item_id | 

### Response:

select product_id from invoices group by product_id order by count ( * ) desc limit 1