"rows"
             $rv = $sth->rows;

           Returns the number of rows affected by the last row affecti
+ng com-
           mand, or -1 if the number of rows is not known or not avail
+able.

           Generally, you can only rely on a row count after a non-"SE
+LECT"
           "execute" (for some specific operations like "UPDATE" and
           "DELETE"), or after fetching all the rows of a "SELECT" sta
+tement.

           For "SELECT" statements, it is generally not possible to kn
+ow how
           many rows will be returned except by fetching them all.  So
+me
           drivers will return the number of rows the application has 
+fetched
           so far, but others may return -1 until all rows have been f
+etched.
           So use of the "rows" method or $DBI::rows with "SELECT" sta
+tements
           is not recommended.

           One alternative method to get a row count for a "SELECT" is
+ to exe-
           cute a "SELECT COUNT(*) FROM ..." SQL statement with the sa
+me "..."
           as your query and then fetch the row count from that.
[download]

  my $sql = {
     SELECT count(*)
     FROM patient_data
     WHERE name = ?
  };
  my $sth = $dbh->prepare($sql);
  $sth->execute($name);
  my ($count_rows) = $sth->fetchrow_array();
[download]

  my $sql = {
     SELECT name
     FROM patient_data
     WHERE name = ?
  };
  my $sth = $dbh->prepare($sql);
  $sth->execute($name);
  my $count_rows = 0;
  while (my ($name) = $sth->fetchrow_array()) {
     ### do something with $name here ###
     $count_rows += 1;
  }
[download]

my $sql = {
     SELECT count(*)
     FROM patient_data
     WHERE name = ?
  };
  my $sth = $dbh->prepare($sql);
  $sth->execute($name);
  my ($count_rows) = $sth->fetchrow_array();
[download]

Note that this is not complete without a finish() call:

$sth->finish();
[download]

You need to use finish if you don't repeat calling fetchrow_*, or equivalent, until it returns undef. So you need it in your second example, too.

If you omit it, you'll get a warning from DBI, or you should. (Maybe this depends on the database driver?)

I'm not quite sure what you mean by "proactively", but this deals with the case of no rows found ...

my $dbh = DBI->connect($conn_string, $user, $password,
{
    RaiseError => 1,
}) || die $DBI::errstr;

my $sth = $dbh->prepare(<<__END_SQL__);
SELECT stuff
  FROM some_place
 WHERE some_condition = 1
__END_SQL__

$sth->execute;

my $num_rows = 0;
while (my @x = $sth->fetchrow)
{
    $num_rows++;
    do_something_with(@x);
}

die "No rows found\n" unless $num_rows;

print "Done\n";
[download]

------
We are the carpenters and bricklayers of the Information Age.

The idea is a little like C++ templates, except not quite so brain-meltingly complicated. -- TheDamian, Exegesis 6

... strings and arrays will suffice. As they are easily available as native data types in any sane language, ... - blokhead, speaking on evolutionary algorithms

Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.

I fully agree. I had the same problems and scanned the web quite extensively. The SELECT COUNT method seems to work best (well, at least for me :) I'd prefer it over fetching all rows and counting them in a while loop, since that seems like a waste of resources to me (especially on really large data sets), and also over the $sth->rows, 'cause that doesn't seem to be reliable on SELECT queries.

--
B10m
Yes, that is one long line ;)