1Z0-051 QUESTION 14 DISTINCT的用法

Using the CUSTOMERS table, you need to generate a report that shows 50% of each credit amount in each income level. The report should NOT show any repeated credit amounts in each income level.
Which query would give the required result?
A. SELECT cust_income_level, DISTINCT cust_credit_limit * 0.50 AS "50% Credit Limit"
    FROM customers;
B. SELECT DISTINCT cust_income_level, DISTINCT cust_credit_limit * 0.50 AS "50% Credit Limit"
    FROM customers;
C. SELECT DISTINCT cust_income_level ||' '|| cust_credit_limit * 0.50 AS "50% Credit Limit"
    FROM customers;
D. SELECT cust_income_level|| ' ' ||cust_credit_limit * 0.50 AS "50% Credit Limit"

    FROM customers;

答案：C

解析：

--A选项报错
SQL> SELECT cust_income_level, DISTINCT cust_credit_limit * 0.50 AS "50% Credit
Limit"
  2      FROM customers;
SELECT cust_income_level, DISTINCT cust_credit_limit * 0.50 AS "50% Credit Limit
"
                          *
ERROR at line 1:
ORA-00936: missing expression

--B选项报错
SQL> SELECT DISTINCT cust_income_level, DISTINCT cust_credit_limit * 0.50 AS "50
% Credit Limit"
  2      FROM customers;
SELECT DISTINCT cust_income_level, DISTINCT cust_credit_limit * 0.50 AS "50% Cre
dit Limit"
                                   *
ERROR at line 1:
ORA-00936: missing expression

--C选项得出正确的结果
SQL> SELECT DISTINCT cust_income_level ||' '|| cust_credit_limit * 0.50 AS "50%
Credit Limit"
  2      FROM customers;

50% Credit Limit
--------------------------------------------------------------------------------

level4 2500
level12 25000
level2 25000
level3 250000
level1 2500
--D选项把重复的结果都显示出来了
SQL> SELECT cust_income_level|| ' ' ||cust_credit_limit * 0.50 AS "50% Credit Li
mit"
  2      FROM customers;

50% Credit Limit
--------------------------------------------------------------------------------

level1 2500
level2 25000
level3 250000
level12 25000
level1 2500
level4 2500

6 rows selected.