OCP 11g题库详解：1Z0-051-057题
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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;
Answer: B
试题解析：
AWHERE子句不能跟别名
B正确
C错误，一是where子句不能用别名，二是非null不能用<>来表达
D错误，语法错误
