一、原题
View the Exhibit and examine the structure of the CUSTOMERS table.
Using the CUSTOMERS table,you need to generate a report that shows an increase in the credit limit by 15% for all customers. Customers whose credit limit has not been entered should have the message "Not Available" displayed.
Which SQL statement would produce the required result?
A. SELECT NVL(cust_credit_limit,'Not Available')*.15 "NEW CREDIT"
FROM customers;
B. SELECT NVL(cust_credit_limit*.15,'Not Available') "NEW CREDIT"
FROM customers;
C. SELECT TO_CHAR(NVL(cust_credit_limit*.15,'Not Available')) "NEW CREDIT"
FROM customers;
D. SELECT NVL(TO_CHAR(cust_credit_limit*.15),'Not Available') "NEW CREDIT"
FROM customers;
答案：D
二、题目翻译
下面是CUSTOMERS表的结构：
根据CUSTOMERS表生成一个报表，显示所有客户增长15%的credit limit.如果客户没有credit limit则显示Not Available.
哪个SQL语句给出所需结果？
三、题目解析
A、B、C选项不正确，原因类似，NVL函数第一个参数是number类型，第二个参数是字符类型，无法隐式转换。
D选项正确，先将cust_credit_limit * 0.15，这是数值类型，用to_char转成字符类型，nvl的两个参数类型一致，就正确了。
四、测试
这里，使用oracle已经有的测试表emp，其中comm列有null值,测试如下：
SQL> SELECT NVL(comm,'Not Available')*.15 "NEW CREDIT"
2 FROM emp;
SELECT NVL(comm,'Not Available')*.15 "NEW CREDIT"
*
ERROR at line 1:
ORA-01722: invalid number
SQL> SELECT NVL(comm*.15,'Not Available') "NEW CREDIT"
2 FROM emp;
SELECT NVL(comm*.15,'Not Available') "NEW CREDIT"
*
ERROR at line 1:
ORA-01722: invalid number
SQL> SELECT TO_CHAR(NVL(comm*.15,'Not Available')) "NEW CREDIT"
2 FROM emp;
SELECT TO_CHAR(NVL(comm*.15,'Not Available')) "NEW CREDIT"
*
ERROR at line 1:
ORA-01722: invalid number
SQL> SELECT NVL(TO_CHAR(comm*.15),'Not Available') "NEW CREDIT"
2 FROM emp;
NEW CREDIT
----------------------------------------------------------------
Not Available
45
75
Not Available
210
Not Available
Not Available
Not Available
Not Available
0
Not Available
Not Available
Not Available
Not Available
14 rows selected.