-- Language PostgreSQL
        -- Tables: 
             -- Table: reference payment methods 
columns : [['payment method code', 'text'], ['payment method description', 'text']]

 -- Table: reference service types 
columns : [['service type code', 'text'], ['parent service type code', 'text'], ['service type description', 'text']]

 -- Table: addresses 
columns : [['address id', 'text'], ['line 1', 'text'], ['line 2', 'text'], ['city town', 'text'], ['state county', 'text'], ['other details', 'text']]

 -- Table: products 
columns : [['product id', 'text'], ['product name', 'text'], ['product price', 'number'], ['product description', 'text'], ['other product service details', 'text']]

 -- Table: marketing regions 
columns : [['marketing region code', 'text'], ['marketing region name', 'text'], ['marketing region descriptrion', 'text'], ['other details', 'text']]

 -- Table: clients 
columns : [['client id', 'number'], ['address id', 'number'], ['customer email address', 'text'], ['customer name', 'text'], ['customer phone', 'text'], ['other details', 'text']]

 -- Table: drama workshop groups 
columns : [['workshop group id', 'number'], ['address id', 'number'], ['currency code', 'text'], ['marketing region code', 'text'], ['store name', 'text'], ['store phone', 'text'], ['store email address', 'text'], ['other details', 'text']]

 -- Table: performers 
columns : [['performer id', 'number'], ['address id', 'number'], ['customer name', 'text'], ['customer phone', 'text'], ['customer email address', 'text'], ['other details', 'text']]

 -- Table: customers 
columns : [['customer id', 'text'], ['address id', 'number'], ['customer name', 'text'], ['customer phone', 'text'], ['customer email address', 'text'], ['other details', 'text']]

 -- Table: stores 
columns : [['store id', 'text'], ['address id', 'number'], ['marketing region code', 'text'], ['store name', 'text'], ['store phone', 'text'], ['store email address', 'text'], ['other details', 'text']]

 -- Table: bookings 
columns : [['booking id', 'number'], ['customer id', 'number'], ['workshop group id', 'text'], ['status code', 'text'], ['store id', 'number'], ['order date', 'time'], ['planned delivery date', 'time'], ['actual delivery date', 'time'], ['other order details', 'text']]

 -- Table: performers in bookings 
columns : [['order id', 'number'], ['performer id', 'number']]

 -- Table: customer orders 
columns : [['order id', 'number'], ['customer id', 'number'], ['store id', 'number'], ['order date', 'time'], ['planned delivery date', 'time'], ['actual delivery date', 'time'], ['other order details', 'text']]

 -- Table: order items 
columns : [['order item id', 'number'], ['order id', 'number'], ['product id', 'number'], ['order quantity', 'text'], ['other item details', 'text']]

 -- Table: invoices 
columns : [['invoice id', 'number'], ['order id', 'number'], ['payment method code', 'text'], ['product id', 'number'], ['order quantity', 'text'], ['other item details', 'text'], ['order item id', 'number']]

 -- Table: services 
columns : [['service id', 'number'], ['service type code', 'text'], ['workshop group id', 'number'], ['product description', 'text'], ['product name', 'text'], ['product price', 'number'], ['other product service details', 'text']]

 -- Table: bookings services 
columns : [['order id', 'number'], ['product id', 'number']]

 -- Table: invoice items 
columns : [['invoice item id', 'number'], ['invoice id', 'number'], ['order id', 'number'], ['order item id', 'number'], ['product id', 'number'], ['order quantity', 'number'], ['other item details', 'text']]

You are a SQL code translator. You have been given the Table data above. Your role is to translate natural language to PostgreSQL. 
You should not select columns that are not part of the tables provided to you. Think step by step.
Your only output should be SQL code. Do not include any other text. Only SQL code.



Question: What is the product ID of the most frequently ordered item on invoices?
SQL: SELECT Product_ID FROM INVOICES GROUP BY Product_ID ORDER BY COUNT(*) DESC LIMIT 1