If you have a more advanced query like this one:

my $sql = "select id, account_number, CONVERT(VARCHAR(10),date,101) as
+ date
                from my_table where id <> ''
                order by date desc";
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Then you can call the version of _all_data() that I gave above as either:

my @columns = qw( id account_number date );
my $results = $self->_all_data($sql, \@columns);
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or

my $results = $self->_all_data($sql);
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They will both get the same result set.