Syntax:

<code>
[[LEFT | INNER] JOIN <component_reference1>] [ON | WITH] <join_condition>, 
[[LEFT | INNER] JOIN <component_reference2>] [ON | WITH] <join_condition>,
... 
[[LEFT | INNER] JOIN <component_referenceN>] [ON | WITH] <join_condition>
</code>

DQL supports two kinds of joins INNER JOINs and LEFT JOINs. For each joined component, you can optionally specify an alias.

* The default join type is {{LEFT JOIN}}. This join can be indicated by the use of either {{LEFT JOIN}} clause or simply '{{,}}', hence the following queries are equal:

<code>
SELECT u.*, p.* FROM User u LEFT JOIN u.Phonenumber

SELECT u.*, p.* FROM User u, u.Phonenumber p
</code>

The recommended form is the first one.

* {{INNER JOIN}} produces an intersection between two specified components (that is, each and every record in the first component is joined to each and every record in the second component). So basically {{INNER JOIN}} can be used when you want to efficiently fetch for example all users which have one or more phonenumbers.

<code>
SELECT u.*, p.* FROM User u INNER JOIN u.Phonenumber p
</code>

By default DQL auto-adds the primary key join condition, so for DQL query:

<code>
SELECT u.id, p.id FROM User u LEFT JOIN u.Phonenumber
</code>

Would have a SQL equivalent:

<code>
SELECT u.id AS u__id, p.id AS p__id FROM User u LEFT JOIN Phonenumber p ON u.id = p.user_id
</code>

If you want to override this behaviour and add your own custom join condition you can do it with the {{ON}} keyword. Consider the following DQL query:

<code>
SELECT u.id, p.id FROM User u LEFT JOIN u.Phonenumber ON u.id = 2
</code>

This query would be converted into SQL:

<code>
SELECT u.id AS u__id, p.id AS p__id FROM User u LEFT JOIN Phonenumber p ON u.id = 2
</code>

Most of the time you don't need to override the primary join condition, rather you may want to add some custom conditions. This can be achieved with the {{WITH}} keyword.

DQL:
<code>
SELECT u.id, p.id FROM User u LEFT JOIN u.Phonenumber WITH u.id = 2
</code>

SQL:
<code>
SELECT u.id AS u__id, p.id AS p__id FROM User u LEFT JOIN Phonenumber p ON u.id = p.user_id AND u.id = 2
</code>

The Doctrine_Query API offers two convenience methods for adding JOINS. These are called innerJoin() and leftJoin(), which usage should be quite intuitive as shown below:

<code type="php">
$q = new Doctrine_Query();
$q->from('User u')
  ->leftJoin('u.Group g')
  ->innerJoin('u.Phonenumber p WITH u.id > 3')
  ->leftJoin('u.Email e');

$users = $q->execute();
</code>

