How can I fetch a single count value from a database with DBI?

问题

The following code seems to be just too much, for getting a single count value. Is there a better, recommended way to fetch a single COUNT value using plain DBI?

sub get_count {
   my $sth = $dbh->prepare("SELECT COUNT(*) FROM table WHERE...");
   $sth->execute( @params );
   my $($count) = $sth->fetchrow_array;
   $sth->finish;

   return $count;
}

This is shorter, but I still have two statements.

sub get_count_2 {
   my $ar = $dbh->selectall_arrayref("SELECT ...", undef, @params)
   return $ar->[0][0];
}

回答1:

Easy enough to do in one line with no extra variables:

$count = $dbh->selectrow_array('SELECT count(*) FROM table WHERE...', undef, @params);

回答2:

I don't know Perl, but if it's syntax is logical I would think this would work based on your 2nd example:

sub get_count {
   return $dbh->selectall_arrayref("SELECT ...", undef, @params)->[0][0];
}

回答3:

I probably wouldn't do this myself, but you could always make it a new top-level function of the DBH object you're using:

WARNING: untested code follows!

sub DBD::SQLite::db::count
{
   my($dbh, $table, $where) = @_;

   my($stmt) = "SELECT COUNT(*) FROM $table";
   $stmt .= " WHERE $where" if $where;

   my($count) = $dbh->selectrow_array($stmt);

   return $count;

}

and then call it like this:

my($cnt) = $dbh->count('Employee', 'year_hired < 2000');

Besides polluting a namespace that's not yours, you'd also have to write this for every DB driver you use, though I'm sure your could work something up that allows you to construct and eval some code to auto-configure this for a given DBH object.

来源：https://stackoverflow.com/questions/1656748/how-can-i-fetch-a-single-count-value-from-a-database-with-dbi

标签

database

perl

dbi