Just for fun, here is a completely runnable abstract example that doesn't use SQL at all, we are instead iterating over various lists that sort of simulate selecting a single column from several joined tables (No, you probably wouldn't really want to do this sort of hard coded list iterator this way, but it would be a good way to splice in hardcoded lists as sort of a Cartesian join on some SQL when combined with the SQL iterators):

use strict;
use warnings;

use DBIx::Iterator qw(mk_iterator list_iterator);

my $iter = mk_iterator(
  \&list_123,
  [
    \&list_abc,
    \&list_def,
  ],
  [
    \&list_456,
    \&list_789,
  ],
);

use Data::Dumper qw(Dumper);
while ( my $r = $iter->() ) {
  print Dumper($r);
}

sub list_ab {
  my $x = shift;
  my $y = shift;
  list_iterator(
    LIST => [ map {[$_]} $x..$y ],
    SELECT => [ "$x$y" ],
    @_,
 )
}

sub list_123 { list_ab(1,3,@_) }
sub list_456 { list_ab(4,6,@_) }
sub list_789 { list_ab(7,9,@_) }
sub list_abc { list_ab("a","c",@_) }
sub list_def { list_ab("d","f",@_) }
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