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   Indicates that the indexkey expression is to be unique. Different from the
   Primary Key (if not specified otherwise), the fields that make up a Unique
   Key can be accessed even when modifying. During data entry, a check is
   activated that will prevent the entering of duplicate key values.

   The index defined as Unique Key is characterised by U.

Attention - A Unique Key is not the same as the UNIQUE
   attribute of the index itself, which allows duplicate entries to be stored
   in the file, and does not check the data entered.