101.Examine the data in the PROMO_BEGIN_DATE column of the PROMOTIONS table:

101.Examine the data in the PROMO_BEGIN_DATE column of the PROMOTIONS table:

PROMO_BEGIN_DATE

04-jan-00

10-jan-00

15-dec-99

18-oct-98

22-aug-99

You want to display the number of promotions started in 1999 and 2000.

Which query gives the correct output?

A.SELECT SUM(DECODE(SUBSTR(promo_begin_date,8),'00',1,0)) "2000", SUM(DECODE(SUBSTR(promo_begin_date,8),'99',1,0)) "1999" FROM promotions;

B.SELECT SUM(CASE TO_CHAR(promo_begin_date,'yyyy') WHEN '99' THEN 1 ELSE 0 END) "1999",SUM(CASE TO_CHAR(promo_begin_date,'yyyy') WHEN '00' THEN 1 ELSE 0 END) "2000" FROM promotions;

C.SELECT COUNT(CASE TO_CHAR(promo_begin_date,'yyyy') WHEN '99' THEN 1 ELSE 0 END) "1999",COUNT(CASE TO_CHAR(promo_begin_date,'yyyy') WHEN '00' THEN 1 ELSE 0 END) "2000" FROM promotions;

D.SELECT COUNT(DECODE(SUBSTR(TO_CHAR(promo_begin_date,'yyyy'), 8), '1999', 1, 0)) "1999", COUNT(DECODE(SUBSTR(TO_CHAR(promo_begin_date,'yyyy'), 8),'2000', 1,

0)) "2000" FROM promotions;

答案：A

解析：

A:正确，SUBSTR(promo_begin_date,8)这个返回yy格式的年，如果是99返回1否则返回0，然后对整个记录执行sum，后面2000年那个也一样，因此正确

B:错误，TO_CHAR(promo_begin_date,'yyyy')这个返回的是yyyy这种格式，与99不匹配

C:错误，和B一样的错误

D:错误，TO_CHAR(promo_begin_date,'yyyy')返回的是个yyyy的年，然后SUBSTR对这个四位的年进行截取，截取位置为第八位,大于了总长度，返回null