The following UPDATE
statement will guarantee that each row has a unique MY_TIMESTAMP
value, by increasing the milliseconds by the rownum value.
EDIT:
After Alessandro Rossi pointed out that there could be duplicate values, the following query has been modified to use SYSTIMESTAMP for the update.
UPDATE ITEM_HISTORY
SET my_timestamp = SYSTIMESTAMP + NUMTODSINTERVAL(rownum/1000, 'SECOND');
However, it is recommended that you use an alternative strategy, such as adding another column to store the key value.
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solved Oracle Sql Statement for unique timestamp for each row