OCP-1Z0-051-題目解析-第14題

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? 
（題意：使用客戶表，需要做一個報表，列出每個收入等級的信用額度的50%，並去除重複值。下面的哪個查詢語句可以得到這個結果？）
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

題目解析：

這道題是關於distinct的用法：
1.distinct只能放在第一個欄位的前面，如Select distinct x,y from t,
2.當distinct後有多個欄位時，表示所有欄位的值都相同才視為重複值，如Select distinct x,y from t,
  只有當x，y的值都相同時，才視為重複值去除。

所以選項AB是語法錯誤，選項D沒有去除重複值，不合題意。
其實C也有點錯誤，正確的寫法是：

SELECT DISTINCT cust_income_level || cust_credit_limit * 0.50 AS "50% Credit Limit" FROM customers; 

語句中的''應該換成串連符||