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

1  –  cbd_quoted_string

  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.

2  –  Usage Notes

   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.

3  –  Example

  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".