The value of the IDENTITY column uniquely identifies each row in a table. However, if a table’s select statement contains a union or join, individual rows can appear multiple times in the result set. An IDENTITY column that is selected into a table with a union or join does not retain the IDENTITY property. If the table contains the union of the IDENTITY column and a NULL column, the new column is defined as NULL. Otherwise, it is NOT NULL.
For more information, see “Using IDENTITY columns” and “Updating IDENTITY columns”. See also select in the Reference Manual.
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