一、scn的作用
Scn在数据库中无处不在，一般在控制文件，数据文件头，日志文件，数据文件等都记录scn值。
system scn从视图v$database中获得，对应checkpoint_change#字段；
datafile scn、last scn（stop scn）分别对应视图v$datafile中的checkpoint_change#，last_change#，
start scn则从v$datafile_header中checkpoint_change#得到。
二、查看当前scn
control中有三种SCN分别为：system SCN、datafile SCN、last SCN（stop scn），
System scn
SQL> select checkpoint_change# from v$database;
CHECKPOINT_CHANGE#
------------------
451490
Datafile scn
SQL> select name,checkpoint_change# from v$datafile;
NAME
--------------------------------------------------------------------------------
CHECKPOINT_CHANGE#
------------------
/opt/app/oracle/oradata/wolf/system01.dbf
451490
/opt/app/oracle/oradata/wolf/undotbs01.dbf
451490
/opt/app/oracle/oradata/wolf/sysaux01.dbf
451490
NAME
--------------------------------------------------------------------------------
CHECKPOINT_CHANGE#
------------------
/opt/app/oracle/oradata/wolf/users01.dbf
451490
Last scn（stop scn，正常启动的时候stop scn无穷大， last_change#一定是NULL）
SQL> select name,last_change# from v$datafile;
NAME
--------------------------------------------------------------------------------
LAST_CHANGE#
------------
/opt/app/oracle/oradata/wolf/system01.dbf
/opt/app/oracle/oradata/wolf/undotbs01.dbf
/opt/app/oracle/oradata/wolf/sysaux01.dbf
NAME
--------------------------------------------------------------------------------
LAST_CHANGE#
------------
/opt/app/oracle/oradata/wolf/users01.dbf
数据文件头中有一种SCN ：start SCN
SQL> select name,checkpoint_change# from v$datafile_header;
NAME
--------------------------------------------------------------------------------
CHECKPOINT_CHANGE#
------------------
/opt/app/oracle/oradata/wolf/system01.dbf
451490
/opt/app/oracle/oradata/wolf/undotbs01.dbf
451490
/opt/app/oracle/oradata/wolf/sysaux01.dbf
451490
NAME
--------------------------------------------------------------------------------
CHECKPOINT_CHANGE#
------------------
/opt/app/oracle/oradata/wolf/users01.dbf
451490