Show the transaction type code that occurs the fewest times.
select transaction_type_code from transactions group by transaction_type_code order by count(*) asc limit 1
Show the number of occurances of each transaction type code.
select transaction_type_code, count(*) from transactions group by transaction_type_code
Which transaction occurs the most frequently?
select * from transactions group by transaction_type_code order by count(*) desc limit 1
Show its transaction type code.
select transaction_type_code from transactions group by transaction_type_code order by count(*) desc limit 1