Commit 1350eaea authored by zYne's avatar zYne

--no commit message

--no commit message
parent a269fcdb
......@@ -3,6 +3,7 @@
++ UPDATE queries
++ DELETE queries
++ FROM clause
++ JOIN syntax
++ WHERE clause
++ Conditional expressions
++ Functional Expressions
......@@ -11,4 +12,4 @@
++ ORDER BY clause
++ LIMIT and OFFSET clauses
++ Examples
++ BNF
++ BNF
......@@ -8,14 +8,6 @@ The {{FROM}} clause indicates the component or components from which to retrieve
* 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>
* {{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>
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>
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment