14. 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?
（题意：使用客户表，须要做一个报表，列出每一个收入等级的信用额度的50%，并去除反复值。以下的哪个查询语句能够得到这个结果？）
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;
Answer: C
题目解析：
这道题是关于distinct的使用方法：
1.distinct仅仅能放在第一个字段的前面，如Select distinct x,y from t,
2.当distinct后有多个字段时，表示全部字段的值都同样才视为反复值，如Select distinct x,y from t,
仅仅有当x，y的值都同样时，才视为反复值去除。
所以选项AB是语法错误，选项D没有去除反复值，不合题意。
事实上C也有点错误，正确的写法是：
SELECT DISTINCT cust_income_level || cust_credit_limit * 0.50
AS "50% Credit Limit"
FROM customers;
语句中的''应该换成连接符||