OCP-1Z0-051 第104题 DECODE,NVL,COALESCE函数用法

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一、原题

Examine the structure of the PROMOS table:
Name                      Null              Type
PROMO_ID                NOT NULL        NUMBER(3)
PROMO_NAME                              VARCHAR2(30)
PROMO_START_DATE        NOT NULL        DATE
PROMO_END_DATE                          DATE
You want to generate a report showing promo names and their duration (number of days). If the PROMO_END_DATE has not been entered, the message 'ONGOING' should be displayed.
Which queries give the correct output? (Choose all that apply.)
A.SELECT promo_name,TO_CHAR(NVL(promo_end_date-promo_start_date,'ONGOING'))
       FROM promos;

B. SELECT promo_name,COALESCE(TO_CHAR(promo_end_date-promo_start_date),'ONGOING')
       FROM promos;

C.SELECT promo_name,NVL(TO_CHAR(promo_end_date-promo_start_date),'ONGOING')
      FROM promos;

D. SELECT promo_name,DECODE(promo_end_date-promo_start_date,NULL,'ONGOING',
                                                          promo_end_date-promo_start_date)
        FROM promos;

E. SELECT promo_name,decode(coalesce(promo_end_date,promo_start_date),null,'ONGOING',
                                                       promo_end_date - promo_start_date))
       FROM promos;

答案：BCD

二、题目翻译

查看表结构
显示promo names和their duration，如果PROMO_END_DATE没有值，则显示'ONGOING'。
哪个查询能得出正确的结果(选择所有正确选项)？

三、题目解析

A选项不正确，因为隐式转换'ONGOING'为数值类型的会报错。
E选项不正确，不能输出正确的结果。

NVL和NULLIF，COALESCE的用法，详见：
http://blog.csdn.net/holly2008/article/details/25251513

DECODE的用法，详见：
http://blog.csdn.net/holly2008/article/details/23140591

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