What is the average checking balance?
select avg(balance) from checking
What is the account information for those have greater checking balances than that?
select * from accounts as t1 join checking as t2 on t1.custid = t2.custid where t2.balance > (select avg(balance) from checking)
How about for lower checking balances?
select * from accounts as t1 join checking as t2 on t1.custid = t2.custid where t2.balance < (select avg(balance) from checking)
What are their account names?
select t1.name from accounts as t1 join checking as t2 on t1.custid = t2.custid where t2.balance < (select avg(balance) from checking)