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8 %unique and %key

These special field names are used to avoid several instances of the same field in a record, and to implement keys in record sets. Their usage is:

     %unique: field1 field2 ... fieldN
     %key: field

The list of field names are separated by one or more blank characters.

The %unique special field allows to declare fields as unique, meaning there cannot exist more than one field with the same name in a single record.

For example, an entry in an addressbook database could contain an Age field. It does not make sense for a single person to be of several ages, so that field could be declared as “unique” in the corresponding record descriptor as follows:

     %rec: Contact
     %mandatory: Name
     %unique: Age

Several %unique fields can appear in the same record descriptor. The set of unique fields is the union of all the entries.

%key makes the referred field the primary key of the record set. Its effect is that any field with that name must be both unique and mandatory, and additionally the values of those fields shall be unique in the context of the record set. This closely corresponds to the notion of “primary key” usually implemented in the relational systems.

Consider for example a database of items in a stockage. Each item is idenfitied by a numerical Id field. No item may have more than one Id, and no items may exist without an associated Id. Additionally, no two items may share the same Id. This common situation can be implementing by declaring Id as the key in the record descriptor:

     %rec: Item
     %key: Id
     %mandatory: Title
     Id: 1
     Title: Box
     Id: 2
     Title: Sticker big

It would not make sense to have several primary keys in a record set, and thus it is not allowed to have several %key fields in the same record descriptor. That situation is considered a data integrity violation and will be reported by a checking tool.