Which three statements are true about sequences in a single instance Orade database?
A) A sequence‘s unallocated cached values are lost if the instance shuts down.
B) Two or more tables cannot have keys generated from the same sequence.
C) A sequence number that was allocated can be rolled back if a transaction fails.
D) A sequence can issue duplicate values.
E) Sequences can always have gaps.
F) A sequence can only be dropped by a DBA.
Answer：：ADE
(解析：虚拟可以一下子产生一组的数字缓存在内存中，但是实例关闭，没有用到的数字也将丢失;序列可以产生重复的值在定义的时候用 cycle 关键字，步长如果不是定义为 1，则产生的数字就不是连续的。)
原文：https://blog.51cto.com/13854012/2453157