my $limit = 1;
my $query = qq{SELECT id FROM table ORDER BY RAND() LIMIT $limit};
my $ids = $dbh->selectcol_arrayref($query)
    or die "execution error ($DBI::errstr\n";
$query = qq{SELECT question, answer FROM table
    WHERE id in (}
    . (join ",", @$ids)
    . " ) " ;