MySQL: Add sequence column based on another field

Question

I\'m working on some legacy code/database, and need to add a field to the database which will record a sequence number related to that (foreign) id.

Example table da

Answer 1

The question is tagged as "mysql", so yes, MySQL's auto_increment can create groupwise sequential ids.
see http://dev.mysql.com/doc/refman/5.0/en/example-auto-increment.html:

For MyISAM and BDB tables you can specify AUTO_INCREMENT on a secondary column in a multiple-column index. In this case, the generated value for the AUTO_INCREMENT column is calculated as MAX(auto_increment_column) + 1 WHERE prefix=given-prefix. This is useful when you want to put data into ordered groups.

edit: example php script (using PDO, but it's the same game with the php-mysql module)

$pdo = new PDO('mysql:host=...;dbname=...', '...', '...'); 
$pdo->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION );

// example table
$pdo->exec(
  'CREATE TEMPORARY TABLE Foo (
    id int auto_increment,
    account int,
    someotherstuff varchar(32),
    primary key(account,id)
  ) engine=MyIsam'
);
// insert example data
$stmt = $pdo->prepare('INSERT INTO Foo (account,someotherstuff) VALUES (?,?)');
$stmt->execute(array(1, '1a'));
$stmt->execute(array(1, '1b'));
$stmt->execute(array(1, '1c'));
$stmt->execute(array(2, '2a'));
$stmt->execute(array(2, '2b'));
$stmt->execute(array(1, '1d'));
unset($stmt);

// query data
foreach( $pdo->query('SELECT account,id,someotherstuff FROM Foo') as $row ) {
  echo $row['account'], ' ', $row['id'], ' ', $row['someotherstuff'], "\n";
}

prints

1 1 1a
1 2 1b
1 3 1c
2 1 2a
2 2 2b
1 4 1d