What is the average transaction amount?
select avg(amount_of_transaction) from transactions
How many transactions have an amount bigger than 10000?
select count(amount_of_transaction) from transactions where amount_of_transaction > 10000
Please show the purchase details of these transactions.
select t1.purchase_details from purchases as t1 join transactions as t2 on t1.purchase_transaction_id = t2.transaction_id where t2.amount_of_transaction > 10000