The CUSTOMERS table has the following structure:

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;

Answer：B

(解析：條件語句中不能用別名當做條件，該題的題意是稅額不能為 0，所以只要 cust_credit_limit 不為 0，則保證稅額不會為 0

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