That is a good thought. Something like this would be fine if the database supported trasactions (which MySQL does not). You would then create the table with or without AUTO_INCREMENT. For MySQL we return the required NULL value for id otherwise we generate one in an atomic select/update/commit/rollback.

use DBI;

my $DB_TYPE = 'mysql';
my $AUTO_COMMIT = 1;
my $DB = 'test';
my $DB_USERNAME = 'user';
my $DB_PASSWORD = 'pwd;

my $dbh = DBI->connect( "dbi:$DB_TYPE:$DB", $DB_USERNAME, $DB_PASSWORD
+, {AutoCommit => $AUTO_COMMIT} ) or die_nice( DBI->errstr() );

END { $dbh->disconnect }

# do_sql('DROP TABLE unique_id'); # for debugging

my $sql =<<SQL;
CREATE TABLE unique_id (
    id                  INT UNSIGNED NOT NULL,
    address_book        INT UNSIGNED NOT NULL,
    task_list           INT UNSIGNED NOT NULL,
    PRIMARY KEY (id) )
SQL

# create unique id table;
do_sql($sql);

# initialize table;
$sql = 'INSERT INTO unique_id VALUES (?,?,?)';
do_sql($sql, 1, 0, 0);

print get_unique_id('address_book'), "\n" for 1..10;

sub get_unique_id {
    my ( $table_name ) = @_;
  return 'NULL' if $DB_TYPE = 'mysql';
    my ($select_sth, $update_sth);    
    my $select_sth = get_sth("SELECT $table_name from unique_id where 
+id = 1" ); 
    my $update_sth = get_sth("UPDATE unique_id SET $table_name = ? whe
+re id = 1"); 
    my $success = 1;
    $select_sth->execute() or $success = 0;
    my ($id) = $select_sth->fetchrow_array  or $success = 0;
    $id++;
    $update_sth->execute($id) or $success = 0;    
    $success = ($success ? $dbh->commit : $dbh->rollback) unless $AUTO
+_COMMIT;
    die_nice( "Couldn't finish get_unique_id transaction: " . $dbh->er
+rstr )  unless $success;
    $select_sth->finish;
    $update_sth->finish;
  return $success ? $id : 0;
}

sub do_sql {
    my $sql = shift;
    my $sth = $dbh->prepare($sql) or 
        die_nice( "Could not prepare SQL statement\n\n$sql\n" . $dbh->
+errstr() );
    $sth->execute(@_) or 
        die_nice( "Could not execute SQL statement\n\n$sql\n" . $sth->
+errstr() );
    $sth->finish;
    $dbh->commit unless $AUTO_COMMIT;
}

sub get_sth {
    my $sql = shift;
    my $sth = $dbh->prepare_cached($sql) or 
        die_nice( "Could not prepare SQL statement\n\n$sql\n" . $dbh->
+errstr() );
  return $sth;
}

sub die_nice {
    die shift;
}
[download]

cheers

tachyon

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