The action of CHANGE RELATION with the DEFINE-CLAUSE depends on
    the define-clause, as follows:

    o  DEFINE global-field-name-includes an existing global field
       definition in the relation

       If you include only a name with the DEFINE option, Oracle Rdb
       searches for a field with that name in the list of global
       field definitions for the database. If such a field definition
       exists, Oracle Rdb adds that field definition to the relation
       definition. If no field has that name, Oracle Rdb returns an
       error message.

    o  DEFINE local-field-name BASED ON global-field-name-includes
       an existing field definition in the relation, but gives it a
       local name

       If you include the BASED ON qualifier, Oracle Rdb uses the
       definition specified by the global-field-name syntax.
       However, the name of the new field in the relation is the
       one specified by the local-field-name syntax. This local name
       does not become part of the global list of field names for the
       database.

       You can specify local DATATRIEVE support clauses for the field
       specified with the local-field-name syntax. If you do, this
       local specification overrides the DATATRIEVE clauses attached
       to the global-field-name.

    o  DEFINE local-field-name COMPUTED BY expression-adds a new
       virtual field

       The COMPUTED BY qualifier causes Oracle Rdb to calculate the
       field's value at run time, based on the specified expression.

       You can specify local DATATRIEVE support clauses on any of
       these fields. For more information on the DATATRIEVE clauses,
       ask for HELP on Field_attr.

    o  If you include the DEFINE CONSTRAINT clause, Oracle Rdb defines
       the specified relation-specific constraint.