QUESTION NO: 21
Which two statements are true about sequences created in a single instance database? (Choose two.)
A. CURRVAL is used to refer to the last sequence number that has been generated
B. DELETE <sequencename> would remove a sequence from the database
C. The numbers generated by a sequence can be used only for one table
D. When the MAXVALUE limit for a sequence isreached, 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: A,D
答案解析：
参考：45：http://blog.csdn.net/rlhua/article/details/12780395
Explanation:
Gaps in the Sequence
Although sequence generators issue sequential numbers without gaps, this action occurs
independent of a commit or rollback. Therefore, if you roll back a statement containing a
sequence, the number is lost.
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Another event that can cause gaps in the sequence is a system crash. If the sequence caches
values in memory, those values are lost if the system crashes.
Because sequences are not tied directly to tables, the same sequence can be used for multiple
tables.
However, if you do so, each table can contain gaps in the sequential numbers.
Modifying a Sequence
If you reach the MAXVALUE limit for your sequence, no additional values from the sequence are
allocated and you will receive an error indicating that the sequence exceeds the MAXVALUE. To
continue to use the sequence, you can modify it by using the ALTER SEQUENCE statement
To remove a sequence, use the DROP statement:
DROP SEQUENCE dept_deptid_seq;