The  COMPUTED  BY  DATATRIEVE   field   attribute   clause   supplies
 expressions used by VAX DATATRIEVE to calculate the values of VIRTUAL
 fields.

Additional information available:

cbd quoted stringUsage NotesExample

 An expression used by VAX DATATRIEVE to calculate the field's value.

 The set of quoted strings must form a valid  VAX  DATATRIEVE  virtual
 expression.   The CDDL compiler does not check the virtual expression
 for correct syntax.  You must provide a correct virtual expression.

  o  You must specify a VIRTUAL FIELD DATATYPE for a field  using  the
     COMPUTED BY DATATRIEVE clause.

  o  Only  VAX  DATATRIEVE  can  interpret  VIRTUAL   fields.    Other
     processors  either  ignore  their  presence  or refuse to process
     record descriptions containing them.

  o  You can use 8-bit characters in CDDL quoted strings.

  o  You cannot specify an INITIAL VALUE field attribute for a virtual
     field.

  o  You cannot specify a CONDITION NAME field attribute for a virtual
     field.

  o  You cannot use a virtual field as a tag variable in the  VARIANTS
     OF field description statement.

 No storage has been allocated here  for  TOTAL_PRICE.   Instead,  VAX
 DATATRIEVE  uses  the  virtual  expression "UNIT_PRICE * QUANTITY" to
 calculate the value of TOTAL_PRICE at run time.

 TOTAL_PRICE            DATATYPE IS VIRTUAL FIELD
                        COMPUTED BY DATATRIEVE AS
                            "UNIT_PRICE * QUANTITY".