What is the transaction type code that occurs most often?
select transaction_type_code from transactions group by transaction_type_code order by count ( * ) desc limit 1
What is the transaction type code that occurs least often?
select transaction_type_code from transactions group by transaction_type_code order by count ( * ) limit 1
What is the average of all transactions with the transaction type code "PUR"?
select avg ( amount_of_transaction ) from transactions where transaction_type_code = "PUR"
What is the average of all transactions with the transaction type code "SALE"?
select avg ( amount_of_transaction ) from transactions where transaction_type_code = "SALE"