sub get_category_loop {
  
  my @cat_loop_hrefs = @{
    $dbh->selectall_arrayref(
      sql_category(), 
      {Slice => {}}
    )
  };
  
  for my $cat_href (@cat_loop_hrefs){
    $cat_href->{subject_loop} = 
      get_subject_loop(delete $cat_href->{category_id});
  }
  
  return [@cat_loop_hrefs];

}