Using selectall_hashref as I would selectall_arrayref - perl

Question

I'm doing some exercises to increase my Perl skills and one of them involves connecting to an SQL database, running a query, and returning the results as an array. This is what I have so far:
my $search = $_[0];
our $dbh = DBI->connect{"dbi:mysql:dbname=database", "root", "password")
or die $DBI::errstr;
my $sql = $dbh->selectall_arrayref("SELECT Player from Players_Sport where Sport like '$search'")
or die $DBI::errstr;
my #array = map { $_->[0] } #$sql;
$dbh->disconnect
or warn "Disconnection failed: $DBI::errstr\n";
return #array;
My next step is to change the code to return the results as a hash instead of an array, but I'm not certain how to proceed. I expect to use selectall_hashref, but all the online examples I've found use WHILE. I want to return the results, then create the hash as separate steps.

Answer 1

Using the selectall_hashref method combines prepare, execute and fetchall_arrayref into a single call. It returns a reference to an array containing a reference to an hash for each row of data fetched. To addon to previous comments of responses.
A verbose way of turning your selectall_arrayref call in to a similar hash where ID is your primary key, you could do something like this.
my %rows = ();
for my $i ( 0..$#{$sql} ) {
my ($id, $player, $sport) = #{$sql->[$i]};
$rows{$id} = [ $player, $sport ];
}
And now to access them:
foreach ( sort(keys(%rows)) ) {
print "$_, -> $rows{$_}->[0], -> $rows{$_}->[1]\n";
}
Now using selectall_hashref, your call would be something like this. ID is the primary key here.
my $href = $dbh->selectall_hashref( q/SELECT ID, Player, Sport
FROM Players_Sport/, q/ID/ );
To access your keys you can use in many ways, here is an example:
foreach ( keys %$href ) {
print "$_, $href->{$_}->{Player}, $href->{$_}->{Sport}\n";
}
Your primary key is placed as $_, your individual keys you can be access out of the hash by saying.
$href->{$_}->{Player}
Again it is just like saying:
foreach my $i ( keys %$href ) {
print "$i, $href->{$i}->{Player}, $href->{$i}->{Sport}\n";
}
Each item is looping through $i

Answer 2

When you use selectall_hashref you must tell it what column to use as the unique key. Specify a column you know will have unique values for every row, then it will populate a hash with 'row' objects using the key column values as the key. Each 'row' is itself a hashref with the column names as the keys.
From the DBI page:
$hash_ref = $dbh->selectall_hashref($statement, $key_field);
$hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr);
$hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr, #bind_values);
In all three versions you provide $key_field to identify the column that will be used as the hash key.

Answer 3

Hashes define an association between 2 things, but what is the key, or value in this case? Assuming that you want a 1 as the value:
my %hash = map { $_, 1 } #array;
or,
my %hash = map { $_->[0], 1 } #$sql;