OCP-1Z0-051-V9.02-45题

45. Which two statements are true about sequences created in a single instance database? (Choose

two.)

A. The numbers generated by a sequence can be used only for one table. 

B. DELETE <sequencename> would remove a sequence from the database. 

C. CURRVAL is used to refer to the last sequence number that has been generated.

D. When the MAXVALUE limit for a sequence is reached, you can increase the MAXVALUE limit by using

the ALTER SEQUENCE statement.

E. When a database instance shuts down abnormally, the sequence numbers that have been cached but

not used would be available once again when the database instance is restarted.

Answer: CD

答案解析：

参考：http://blog.csdn.net/rlhua/article/details/12776779

A不正确，因为一个序列生成的值可以用于多个表

B不正确，因为删除序列的语句为DROP SEQUENCE sequencename

C正确，对，CURRVAL是最后的那个值

D正确，对当最大值达到极限时，可以通过ALTER SEQUENCE修改序列来增加最大值

E不正确，因为如果内存非正常关闭，缓存的数会丢失。

如果建立序列时不指定CACHE和NOCACHE，默认值为20，

即一次性从序列里取20个数放入内存，如果内存崩溃，则这20个数就会丢失，再取值时从第21个数开始取值，CACHE设置的最小值为2。