OCP-1Z0-051-V9.02-104题

104. 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;

Answer: BCD

 答案解析：

ANVL要求括号内参数的类型需一致，,NVL(promo_end_date -promo_start_date,'ONGOING')此处类型不一致，一个为数值型，一个为字符型

B, COALESCE 需括号内参数类型需一致，TO_CHAR(promo_end_date - promo_start_date),'ONGOING') ，此处使用to_char转换，两个参数都为字符型，可以比较。正确。

C, NVL(TO_CHAR(promo_end_date -promo_start_date),'ONGOING')，NVL括号内两个参数都为字符型，正确。

D,表达式正确

E,不满足题意，coalesce返回第一个不为空的值，因为promo_start_date肯定不为空，所以即使promo_end_date为空，coalesce也会返回promo_start_dat，所以都会显示promo_end_date - promo_start_date值，即当promo_end_date为空时，还是会显示空（注意，如果你直接指定NULL-sysdate会报转换数据类型错，而在字段里就可以减，并返回空。