64 lines
2.2 KiB
Plaintext
64 lines
2.2 KiB
Plaintext
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Syntax:
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<code>
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[[LEFT | INNER] JOIN <component_reference1>] [ON | WITH] <join_condition>,
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[[LEFT | INNER] JOIN <component_reference2>] [ON | WITH] <join_condition>,
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...
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[[LEFT | INNER] JOIN <component_referenceN>] [ON | WITH] <join_condition>
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</code>
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DQL supports two kinds of joins INNER JOINs and LEFT JOINs. For each joined component, you can optionally specify an alias.
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* 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:
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<code>
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SELECT u.*, p.* FROM User u LEFT JOIN u.Phonenumber
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SELECT u.*, p.* FROM User u, u.Phonenumber p
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</code>
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The recommended form is the first one.
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* {{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.
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<code>
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SELECT u.*, p.* FROM User u INNER JOIN u.Phonenumber p
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</code>
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By default DQL auto-adds the primary key join condition, so for DQL query:
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<code>
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SELECT u.id, p.id FROM User u LEFT JOIN u.Phonenumber
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</code>
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Would have a SQL equivalent:
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<code>
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SELECT u.id AS u__id, p.id AS p__id FROM User u LEFT JOIN Phonenumber p ON u.id = p.user_id
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</code>
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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:
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<code>
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SELECT u.id, p.id FROM User u LEFT JOIN u.Phonenumber ON u.id = 2
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</code>
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This query would be converted into SQL:
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<code>
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SELECT u.id AS u__id, p.id AS p__id FROM User u LEFT JOIN Phonenumber p ON u.id = 2
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</code>
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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.
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DQL:
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<code>
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SELECT u.id, p.id FROM User u LEFT JOIN u.Phonenumber WITH u.id = 2
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</code>
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SQL:
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<code>
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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
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</code>
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