Question

I need to store the fetched data from the MySQL database. So I used this code.

while (@row = $statement->fetchrow_array) 
{
  print "@row.\n"; #        ------printing data
}

foreach $key(@row)
{
  print $key."\n"; # ------empty data
}

In foreach loop the @row data is empty. How to solve this

UPDATE: It actually should be like this:

while (my @row = $statement->fetchrow_array) { 
  # print "@row.\n";
  foreach my $key(@row) { 
    $query= "ALTER TABLE tablename DROP FOREIGN KEY $key;"; 
    $statement = $connection->prepare($query); 
    $statement->execute() 
      or die "SQL Error: $DBI::errstr\n"; 
  } 
}

Answer 1

Well, it should be like this:

while (my @row = $statement->fetchrow_array) {
  foreach my $key (@row) {
    print $key."\n";
  }
}

Otherwise all the result set will be consumed in the first loop.

As a sidenote, fetchrow_array returns a rowset as array of field values, not keys. To get the keys as well, you should use fetchrow_hashref:

while (my $row = $statement->fetchrow_hashref) {
  while (my ($key, $value) = each %$row) {
    print "$key => $value\n";
  }
}

UPDATE: from your comments it looks like you actually need an array of column names. There’s one way to do it:

my $row = $statement->fetchrow_hashref;
$statement->finish;
foreach my $key (keys %$row) {
  # ... dropping $key from db as in your original example
}

But in fact, there are more convenient ways of doing what you want to do. First, there’s a single method for preparing and extracting a single row: selectrow_hashref. Second, if you want to get just foreign keys information, why don’t use the specific DBI method for that – foreign_key_info? For example:

my $sth = $dbh->foreign_key_info( undef, undef, undef,
                                  undef, undef, $table_name);
my $rows = $sth->fetchall_hashref;
while (my ($k, $v) = each %$rows) {
  # process $k/$v
}