What is the average account balance of customers with credit score below 50 for the different account types?
return account types ;return customers that  have  #1 ;return credits scores of #2 ;return #2 where #3 is  lower than 50 ;return account balances of #4 ;return average of #5 for each #1
['select', 'project', 'project', 'comparative', 'project', 'group']