find the names of customers who have a saving account.
select cust_name from customer where acc_type = 'saving'
how about those who have a checking account?
select cust_name from customer where acc_type = 'checking'
which of them have both kinds of accounts?
select cust_name from customer where acc_type = 'saving' intersect select cust_name from customer where acc_type = 'checking'