Sequences

Description

Sequences are a way of offering unique IDs for data rows. If you do most of your work with e.g. MySQL, think of sequences as another way of doing AUTO_INCREMENT.

It's quite simple, first you request an ID, and then you insert that value in the ID field of the new row you're creating. You can have more than one sequence for all your tables, just be sure that you always use the same sequence for any particular table. To get the value of this unique ID use nextID(), if a sequence doesn't exists, it will be created automatically.

The sequence is automatically incremented each time nextID() is called.

<?php
// Once you have a valid MDB2 object named $mdb2...
$id = $mdb2->nextID('mySequence');
if (PEAR::isError($id)) {
    die($id->getMessage());
}

// Use the ID in your INSERT query
$res =& $mdb2->query("INSERT INTO myTable (id, text) VALUES ($id, 'foo')");
?>

Note

Getting the last inserted ID

If you prefer using AUTO_INCREMENT you can alternatively use the lastInsertID() method to retrieve the last generated value. This method alternatively also supports getting the current ID from a sequence using the format defined in PostgreSQL's SERIAL data type. MDB2 can emulate this behaviour for RDBMS that do not support autoincrement at table creation time when using the createTable() method.

<?php
// Once you have a valid MDB2 object named $mdb2...
$res =& $mdb2->query("INSERT INTO myTable (id, text) VALUES (NULL, 'foo')");

// optionally pass in a table and fieldname in order to call nextID()
// when autoincrement is not supported
$id = $mdb2->lastInsertID('myTable', 'id');
if (PEAR::isError($id)) {
    die($id->getMessage());
}

?>

Getting the current ID

If you can get the current global value of a sequence using the currID() method.

<?php

// getting the current value of a sequence
$id = $mdb2->currID('myseq');
if (PEAR::isError($id)) {
    die($id->getMessage());
}

?>

Getting around emulation

Finally if you prefer using what ever native feature the RDBMS supports you can use the getBeforeID() and getAfterID() methods from the Extended module. This way MDB2 will automatically use AUTO_INCREMENT if its natively supported. If not MDB2 will instead use a sequence to get the next id.

<?php
// Once you have a valid MDB2 object named $mdb2...
// $id may either be a quoted integer or php null
$id = $mdb2->getBeforeID('myTable', 'id', true, true);
if (PEAR::isError($id)) {
    die($id->getMessage());
}

$res =& $mdb2->query("INSERT INTO myTable (id, text) VALUES ($id, 'foo')");

// $id is now equivalent to the value in the id field that was inserted
$id = $mdb2->getAfterID($id, 'myTable', 'id');
if (PEAR::isError($id)) {
    die($id->getMessage());
}

?>

See

createSequence(), dropSequence(), listSequences()