SELECT investor_id ,  avg(amount_of_transaction) FROM TRANSACTIONS GROUP BY investor_id
Show the average amount of transactions for different investors.
[Schema (values) (types)]: | tracking_share_transactions |  Investors : investor_id (text) , investor_details (number) | Lots : lot_id (text) , investor_id (number) , lot_details (text) | Ref_Transaction_Types : transaction_type_code (text) , transaction_type_description (number) | Transactions : transaction_id (text) , investor_id (number) , transaction_type_code (text) , date_of_transaction (number) , amount_of_transaction (number) , share_count (text) , other_details (text) | Sales : sales_transaction_id (text) , sales_details (number) | Purchases : purchase_transaction_id (text) , purchase_details (number) | Transactions_Lots : transaction_id (text) , lot_id (number);