CREATE TABLE Purchases (purchase_transaction_id NUMBER, purchase_details VARCHAR2)
CREATE TABLE Transactions (transaction_id NUMBER, investor_id NUMBER, transaction_type_code VARCHAR2, date_of_transaction DATETIME, amount_of_transaction NUMBER, share_count VARCHAR2, other_details VARCHAR2)
CREATE TABLE Transactions_Lots (transaction_id NUMBER, lot_id NUMBER)
CREATE TABLE Investors (investor_id NUMBER, Investor_details VARCHAR2)
CREATE TABLE Sales (sales_transaction_id NUMBER, sales_details VARCHAR2)
CREATE TABLE Ref_Transaction_Types (transaction_type_code VARCHAR2, transaction_type_description VARCHAR2)
CREATE TABLE Lots (lot_id NUMBER, investor_id NUMBER, lot_details VARCHAR2)

Show the average transaction amount for different transaction types.
SELECT transaction_type_code, AVG(amount_of_transaction) FROM Transactions GROUP BY transaction_type_code
SELECT "transaction_type_code", AVG("amount_of_transaction") FROM "Transactions" GROUP BY "transaction_type_code"