Shell Programming and Scripting

 
  $sth = $dbh->prepare(q{ SELECT region, sales FROM sales_by_region });
  $sth->execute;
  my ($region, $sales);

  # Bind Perl variables to columns:
  $rv = $sth->bind_columns(\$region, \$sales);


  while ($sth->fetch) {
      print "$region: $sales\n";
  }

while ($sth->fetch) {

 if ($region ne "" && $sales ne "") {
      print "$region: $sales\n"; 
 }
 else {
      print "Sorry, result set is empty.\n";
 }

}

$rv = $sth->rows;

Returns the number of rows affected by the last row-affecting command, or -1 if the number of rows is not known or not available.

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

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

One alternative method to get a row count for a SELECT is to execute a "SELECT COUNT(*) FROM ..." SQL statement with the same "..." as your query, and then fetch the row count from that.

$
$
$ # check if the table "sales_by_region" has data in it
$ echo "select * from sales_by_region;" | sqlplus -s test/test
 
no rows selected
 
$
$ # so there's no data in it
$ # display the content of the Perl program "fetchsales.pl"
$ cat -n fetchsales.pl
    1  #!perl
    2  use DBI;
    3  use vars qw($dbh $sth $region $sales $rv);
    4
    5  $dbh = DBI->connect('dbi:Oracle:', 'test', 'test');
    6  $sth = $dbh->prepare(q{ SELECT region, sales FROM sales_by_region });
    7  $sth->execute;
    8  my ($region, $sales);
    9  # Bind Perl variables to columns:
   10  $rv = $sth->bind_columns(\$region, \$sales);
   11  while ($sth->fetch) {
   12    print "$region: $sales\n";
   13  }
   14  $sth->finish();
   15  $dbh->disconnect();
$
$ # run the program
$ perl fetchsales.pl
$
$

$
$
$ # check the data in the "sales_by_region" table
$ echo "select * from sales_by_region;" | sqlplus -s test/test
       ID REGION          SALES
---------- ---------- ----------
        1 East
        2                599.75
        3
        4 South           273.9
 
4 rows selected.
 
$
$
$ # "sales" is NULL for id=1, "region" is NULL for id=2
$ # and both are NULL for id=3
$
$ # display the content of the Perl program "fetchsales.pl"
$ cat -n fetchsales.pl
    1  #!perl
    2  use DBI;
    3  use vars qw($dbh $sth $region $sales $rv);
    4  $dbh = DBI->connect('dbi:Oracle:', 'test', 'test');
    5  $sth = $dbh->prepare(q{ SELECT region, sales FROM sales_by_region });
    6  $sth->execute;
    7  my ($region, $sales);
    8  # Bind Perl variables to columns:
    9  $rv = $sth->bind_columns(\$region, \$sales);
   10  while ($sth->fetch) {
   11    if ($region ne "" && $sales ne "") {
   12      print "$region: $sales\n";
   13    } else {
   14      print "Sorry, result set is empty.\n";
   15    }
   16  }
   17  $sth->finish();
   18  $dbh->disconnect();
   19
$
$ # run the program
$ perl fetchsales.pl
Sorry, result set is empty.
Sorry, result set is empty.
Sorry, result set is empty.
South: 273.9
$
$