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   Two files are in a 1:1 relation when one record of a file A
   corresponds to only one record of file B. The 1:1 relation can be
   considered an extension of file A's structure in the record corresponding
   to file B. File B's record can also not exist, therefore a 1:1 relation
   allows you to save memory when some information is not always available.

   In a data entry system, fields from different files can be edited on the
   same form, only if the files are related with a 1:1 relationship to
   the master file associated to the form.

Example:

   For example, the file Company Registry (A) is in a "One to One"
   relation with the file Administration Data (B) that contains the
   record relative to the company only if that data is available, therefore
   occupying less space on the disk.

   It is possible to set, for the 1:1 relation, specific characteristics
   like:

     . the referential integrity when deleting the primary key on
       the related parent file. You can choose from the 5 types available:
       Restrict, Cascade, Nullify, Orphan and
       Prevent.

     . the primary positioning key that allows to establish the
       positioning criterion of the data file to the first record that
       satisfies the 1:1 relation. Within this field it is possible to
       insert, other than the typical expressions on fields, user functions
       or memory variables that have been set previously in the Symbols
       environment (see symbols), or that have been defined directly from
       this context, by pressing the Ins key. The Alt-1 keys rapidly
       insert the expression of the positioning key.