Which statement adds a constraint that ensures the CUSTOMER_NAME column of the CUSTOMERS table holds a value?()
A:ALTER TABLE customers ADD CONSTRAINT cust_name_nn CHECK customer_name IS NOT NULL;
B:ALTER TABLE customers MODIFY CONSTRAINT cust_name_nn CHECK customer_name IS NOT NULL;
C:ALTER TABLE customers MODIFY customer_name CONSTRAINT cust_name_nn NOT NULL;
D:ALTER TABLE customers MODIFY customer_name CONSTRAINT cust_name_nn IS NOT NULL;
E:ALTER TABLE customers MODIFY name CONSTRAINT cust_name_nn NOT NULL;
F:ALTER TABLE customers ADD CONSTRAINT cust_name_nn CHECK customer_name NOT NULL;
出自:Oracle认证