Fields contained in a redefining item occupy the same positions as the fields being redefined. The compiler needs to select only one of the field definitions to use. The default rule that it follows is to use the fields in the item being redefined as column names; the XFD maps fields that appear subordinate to a REDEFINES clause to column names.
This same rule extends to multiple record definitions. In COBOL, multiple record definitions are essentially redefinitions of the entire record area. This leads to the same complication that is encountered with REDEFINES: multiple definitions for the same data. So the compiler needs to select one definition to use.
Because the multiple record types can be different sizes, the compiler needs to use the largest one, so that it can cover all of the fields adequately. Thus, the compiler's rule is to use the fields in the largest record defined for the file. If more than one record is of the largest size, the compiler uses the first one.
Note that group items are, by default, never included in an XFD for the same reason that REDEFINES are excluded: they result in multiple names for the same data items. You can, however, choose to combine grouped fields into one data item by specifying the USE GROUP XFD directive.
In a COBOL FD, FILLER data items are essentially place holders. FILLER items are not uniquely named and thus cannot be uniquely referenced. For this reason, they are not placed into the XFD. The XFD maintains the correct mapping of the other fields, and no COBOL record positional information is lost.
Sometimes you need to include a FILLER data item, such as when it occurs as part of a key. In such a case, you could include it under a USE GROUP XFD directive or give it a name of its own with the NAME XFD directive.
An OCCURS clause always requires special handling, because the run time system must assign a unique name to each database column. The run time system accomplishes this by appending sequential index numbers to the item named in the OCCURS clause.
For example, if the following were part of a file's description:
03 employee-table occurs 20 times. 05 employee-number pic 9(3)
these column names would be created in the database table:
employee_number_1 employee_number_2 . . . employee_number_10 employee_number_11 . . . employee_number_20
Note that the hyphens in the COBOL code are translated to underscores in database field names, and the index number is preceded by an underscore.