（57）The CUSTOMERS table has the following structure:

name Null Type

CUST_ID NOT NULL NUMBER

CUST_FIRST_NAME NOT NULL VARCHAR2(20)

CUST_LAST_NAME NOT NULL VARCHAR2(30)

CUST_INCOME_LEVEL VARCHAR2(30)

CUST_CREDIT_LIMIT NUMBER

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 IS NOT NULL AND 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,tax_amount) IS NOT NULL;

答案：（B）

解析：

WHERE子句中不允许出现别名

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