65、(22-16) choose the best answer:
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：条件语句中不能用别名当做条件，该题的题意是税额不能为 0，所以只要 cust_credit_limit 不为 0，则保证税额不会为 0