How many transactions are there?
select count(*) from transactions
Show the id and details for the investor who has the most transactions.
select t2.investor_id, t1.investor_details from investors as t1 join transactions as t2 on t1.investor_id = t2.investor_id group by t2.investor_id order by count(*) desc limit 1
How about investors with top 3 number of transactions?
select t2.investor_id, t1.investor_details from investors as t1 join transactions as t2 on t1.investor_id = t2.investor_id group by t2.investor_id order by count(*) desc limit 3