You need to write a query that does the following tasks:
1. Display the first name and tax amount of the customers. Tax is 5% of their credit limit.
2. Only those customers whose income level has a value should be considered.
3. Customers whose tax amount is null should not be considered.

Which statement accomplishes all the required tasks?

A) SELECT cust_first_name, cust_credit_limit * .05 AS TAX_AMOUNT
FROM customers
WHERE (cust_income_level,tax_amount) IS NOT NULL;

B) SELECT cust_first_name, cust_credit_limit * .05 AS TAX_AMOUNT
FROM customers
WHERE cust_income_level IS NOT NULL AND cust_credit_limit IS NOT NULL;

C) SELECT cust_first_name, cust_credit_limit * .05 AS TAX_AMOUNT
FROM customers
WHERE cust_income_level <> NULL AND tax_amount <> NULL;

D) SELECT cust_first_name, cust_credit_limit * .05 AS TAX_AMOUNT
FROM customers WHERE cust_income_level IS NOT NULL AND tax_amount IS NOT NULL;

条件语句，不能用别名做条件。ACD都是用别名做条件。因此排除