Having an SQL SELECT query, how do I get number of items? - perl

Question

I'm writing a web app in Perl using Dancer framework. The database is in sqlite and I use DBI for database interaction.
I'm fine with select statements, but I wonder is there a way to count selected rows.
E.g. I have
get '/' => sub {
my $content = database->prepare(sprintf("SELECT * FROM content LIMIT %d",
$CONTNUM));
$content->execute;
print(Dumper($content->fetchall_arrayref));
};
How do I count all items in the result without issuing another query?
What I want to achieve this way is showing 30 items per page and knowing how many pages there would be. Of course I can run SELECT COUNT (*) foo bar, but it looks wrong and redundant to me. I'm looking for a more or less general, DRY and not too heavy on database way to do so.
Any SQL or Perl hack or a hint what should I read about would be appreciated.
// I know using string concatenation for querys is bad

Answer 1

You have to do it the hard way: one query to get the count and another to get your desired slice of the row set:
my $count = $database->prepare('SELECT COUNT(*) FROM content');
$count->execute();
my $n = $count->fetchall_arrayref()->[0][0];
my $content = $database->prepare('SELECT * FROM content LIMIT ?');
$content->execute($CONTNUM);
#...

Answer 2

Not too familiar with perl, but I assume you can just store the result of $content->fetchall_arrayref and retrieve the count from that array befor you print it.
[edit]
Something like
my $ref = $content->fetchall_arrayref;
my $count = scalar(#$ref);

Answer 3

Don't use sqlite myself but the following might work:
select * from table join (select count(*) from table);
Whether the above works or not the first thing I'd look for is scrollable cursors if you are going to page through results - I doubt sqlite has those. However, in DBI you can use fetchall_arrayref with a max_rows to fetch a "page" at a time. Just look up the example in the DBI docs under fetchall_arrayref - it is something like this:
my $rowcache = [];
while( my $row = ( shift(#$rowcache) || shift(#{$rowcache=$sth->fetchall_arrayref(undef,100)||[]}) )
) {
# do something here
}
UPDATE: Added what you'd get with selectall_hashref assuming the table is called content with one integer column called "a":
$ perl -le 'use DBI; my $h = DBI->connect("dbi:SQLite:dbname=fred.db"); my $r = $h->selectall_hashref(q/select * from content join (select count(*) as count from content)/, "a");use Data::Dumper;print Dumper($r);'
$VAR1 = {
'1' => {
'count' => '3',
'a' => '1'
},
'3' => {
'count' => '3',
'a' => '3'
},
'2' => {
'count' => '3',
'a' => '2'
}
};

Answer 4

If you want to know how many results there will be, as well as getting the results themselves, all in one query, then get the count as a new value:
SELECT COUNT(*) AS num_rows, * from Table WHERE ...
Now the row count will be the first column of every row of your resultset, so simply pop that off before presenting the data.