OCP 11g考试题库详解：1Z0-051-104题
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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
试题解析：
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。
