ORA-08002: sequence string.CURRVAL is not yet defined in this session
Cause: sequence CURRVAL has been selected before sequence NEXTVAL
Action: select NEXTVAL from the sequence before selecting CURRVAL
此错误的意思是当前Session中没有被初始定义的序列sequence string（sequnce string 为自定义序列名称），使用“CURRVAL”保存序列的值，却未成功，因此Oracle抛出该错误。
Error: ORA-08002
Text: sequence string.CURRVAL is not yet defined in this session
Cause: An attempt was made to refer to the current value of a sequence number, before it has been initialized.
Action: Use the sequence number function anytime after the sequence number has been initialized.
1、使用CURRVAL函数之前，必须先调用NEXTVAL函数
2、可能当前Session回滚了，导致序列已被回滚
1、使用NEXTVAL函数初始化序列
2、使用外部变量来保存序列的值，而不是使用CURRVAL函数