Specifically for mysql, you can use a special SQL command: DESCRIBE $table. Yes that works in DBI too. Here's a test table "foo", which I put into the database "test" (present by default): 
CREATE TABLE foo(
  id int(10) unsigned NOT NULL AUTO_INCREMENT,
  letter char(1) NOT NULL,
  name text NOT NULL,
  num float NULL,
  PRIMARY KEY (id)
);

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And here's some demo code checking what we've got: 
#!/usr/local/bin/perl -w
use DBI;
my $dbh = DBI->connect('dbi:mysql:test', "", "", { PrintError => 0 , R
+aiseError => 1});
END { $dbh->disconnect if $dbh }
use Data::Dumper;

my $sth = $dbh->prepare('DESCRIBE foo');
$sth->execute;
while(my $r = $sth->fetchrow_hashref) {
    print Dumper $r;
}

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which shows: 
$VAR1 = {
          'Extra' => 'auto_increment',
          'Field' => 'id',
          'Default' => undef,
          'Key' => 'PRI',
          'Type' => 'int(10) unsigned',
          'Null' => ''
        };
$VAR1 = {
          'Extra' => '',
          'Field' => 'letter',
          'Default' => '',
          'Key' => '',
          'Type' => 'char(1)',
          'Null' => ''
        };
$VAR1 = {
          'Extra' => '',
          'Field' => 'name',
          'Default' => '',
          'Key' => '',
          'Type' => 'text',
          'Null' => ''
        };
$VAR1 = {
          'Extra' => '',
          'Field' => 'num',
          'Default' => undef,
          'Key' => '',
          'Type' => 'float',
          'Null' => 'YES'
        };

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4 records, one for each field. 'Field' contains the field name. In particular, check out the contents of the field 'Key': its value is "PRI" for the primary key, the empty string for the rest.

In reply to Re: DBI determine primary key by bart
in thread DBI determine primary key by Anonymous Monk

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