To prevent the index from accepting duplicate values in key fields, specify the unique option. If the base table on which the index is being created has duplicate values for the key fields of the index, the create index statement fails. Similarly, if an insert or update is attempted that violates the uniqueness constraint of an index created on the table, the insert or update fails. This is true for an update statement that updates multiple rows: the update statement fails when it attempts to write a row that violates the uniqueness constraint.