Determine current SCN of The database using Time and Vice versa

A) Determine current SCN of The database.
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You can query from V$DATABASE or using GET_SYSTEM_CHANGE_NUMBER procedure of DBMS_FLASHBACK package.

1)SQL> SELECT DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER() FROM DUAL;

DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER()
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1053658

2)SQL> SELECT CURRENT_SCN FROM V$DATABASE;

CURRENT_SCN
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1053668


B) Determine current timestamp value.
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SQL> SELECT SYSTIMESTAMP FROM DUAL;

SYSTIMESTAMP
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14-MAY-08 11.00.08.374107 PM -04:00


C) Convert SCN to Timestamp.
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SQL> SELECT SCN_TO_TIMESTAMP(1053639) FROM DUAL;

SCN_TO_TIMESTAMP(1053639)
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14-MAY-08 10.52.15.000000000 PM


D) Convert Timestamp to SCN
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SQL> SELECT TIMESTAMP_TO_SCN('14-MAY-08 11.00.08.374107 PM') FROM DUAL;

TIMESTAMP_TO_SCN('14-MAY-0811.00.08.374107PM')
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1054516