OCP-1Z0-051-题目解析-第14题

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，需要做一个报表，列出每个收入等级的信用额度的50%，并去除重复值。下面的哪个查询语句可以得到这个结果？）
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的用法：
1.distinct只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，表示所有字段的值都相同才视为重复值，如Select distinct x,y from t,
  只有当x，y的值都相同时，才视为重复值去除。

所以选项AB是语法错误，选项D没有去除重复值，不合题意。
其实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||