How can I get the column names when querying with DBI in Perl?

Question

I'm using DBI to query a SQLite3 database. What I have works, but it doesn't return the columns in order. Example:

Query:  select col1, col2, col3, col4 from some_view;
Output:

    col3, col2, col1, col4
    3, 2, 1, 4
    3, 2, 1, 4
    3, 2, 1, 4
    3, 2, 1, 4
    ...

(values and columns are just for illustration)

I know this is happening because I'm using a hash, but how else do I get the column names back if I only use an array? All I want to do is get something like this for any arbitrary query:

    col1, col2, col3, col4
    1, 2, 3, 4
    1, 2, 3, 4
    1, 2, 3, 4
    1, 2, 3, 4
    ...

(That is, I need the output is in the right order and with the column names.)

I'm very much a Perl novice, but I really thought this would be a simple problem. (I've done this before in Ruby and PHP, but I'm having trouble tracking down what I'm looking for in the Perl documentation.)

Here's a pared down version of what I have at the moment:

use Data::Dumper;
use DBI;

my $database_path = '~/path/to/db.sqlite3';

$database = DBI->connect(
  "dbi:SQLite:dbname=$database_path",
  "",
  "",
  {
    RaiseError => 1,
    AutoCommit => 0,
  }
) or die "Couldn't connect to database: " . DBI->errstr;

my $result = $database->prepare('select col1, col2, col3, col4 from some_view;')
    or die "Couldn't prepare query: " . $database->errstr;

$result->execute
    or die "Couldn't execute query: " . $result->errstr;

########################################################################################### 
# What goes here to print the fields that I requested in the query?
# It can be totally arbitrary or '*' -- "col1, col2, col3, col4" is just for illustration.
# I would expect it to be called something like $result->fields
########################################################################################### 

while (my $row = $result->fetchrow_hashref) {
    my $csv = join(',', values %$row);
    print "$csv\n";
}

$result->finish;

$database->disconnect;

Answer 1

You're asking for the result as a hash. A hash is inherently unordered. Perhaps you want fetchrow_arrayref instead.

In fact, if you had looked at keys %$row, you would have seen the corresponding keys being out of order as well. That's the nature of a hash... each key is paired with its value, but the overall ordering of keys or values is optimized for access, not external ordering.

Answer 2

Replace the "what goes here" comment and the following loop with:

my $fields = join(',', @{ $result->{NAME_lc} });
print "$fields\n";

while (my $row = $result->fetchrow_arrayref) {
    my $csv = join(',', @$row);
    print "$csv\n";
}

NAME_lc gives the field names in lowercase. You can also use NAME_uc for uppercase, or NAME for whatever case the database decides to return them in.

You should also probably be using Text::CSV or Text::CSV_XS instead of trying to roll your own CSV file, but that's another question.