Select this check box to indicate that a table will not contain two or more rows with the same value of the index key. The constraint is enforced when rows of the table are updated or new rows are inserted. The constraint is also checked during the execution of the CREATE INDEX statement. If the table already contains rows with duplicate key values, the index is not created.
When the Unique check box is selected, null values are treated as any other values. For example, if the key is a single column that can contain null values, that column can contain no more than one null value.
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