Doctrine Query Language
===========================

DQL stands for Doctrine Query Language and is an Object
Query Language derivate that is very similar to the Hibernate
Query Language (HQL) or the Java Persistence Query Language (JPQL).

In essence, DQL provides powerful querying capabilities over your
object model. Imagine all your objects lying around in some storage
(like an object database). When writing DQL queries, think about
querying that storage to pick a certain subset of your objects.

.. note::

    A common mistake for beginners is to mistake DQL for
    being just some form of SQL and therefore trying to use table names
    and column names or join arbitrary tables together in a query. You
    need to think about DQL as a query language for your object model,
    not for your relational schema.


DQL is case in-sensitive, except for namespace, class and field
names, which are case sensitive.

Types of DQL queries
--------------------

DQL as a query language has SELECT, UPDATE and DELETE constructs
that map to their corresponding SQL statement types. INSERT
statements are not allowed in DQL, because entities and their
relations have to be introduced into the persistence context
through ``EntityManager#persist()`` to ensure consistency of your
object model.

DQL SELECT statements are a very powerful way of retrieving parts
of your domain model that are not accessible via associations.
Additionally they allow to retrieve entities and their associations
in one single SQL select statement which can make a huge difference
in performance in contrast to using several queries.

DQL UPDATE and DELETE statements offer a way to execute bulk
changes on the entities of your domain model. This is often
necessary when you cannot load all the affected entities of a bulk
update into memory.

SELECT queries
--------------

DQL SELECT clause
~~~~~~~~~~~~~~~~~

The select clause of a DQL query specifies what appears in the
query result. The composition of all the expressions in the select
clause also influences the nature of the query result.

Here is an example that selects all users with an age > 20:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM MyProject\Model\User u WHERE u.age > 20');
    $users = $query->getResult();

Lets examine the query:


-  ``u`` is a so called identification variable or alias that
   refers to the ``MyProject\Model\User`` class. By placing this alias
   in the SELECT clause we specify that we want all instances of the
   User class that are matched by this query to appear in the query
   result.
-  The FROM keyword is always followed by a fully-qualified class
   name which in turn is followed by an identification variable or
   alias for that class name. This class designates a root of our
   query from which we can navigate further via joins (explained
   later) and path expressions.
-  The expression ``u.age`` in the WHERE clause is a path
   expression. Path expressions in DQL are easily identified by the
   use of the '.' operator that is used for constructing paths. The
   path expression ``u.age`` refers to the ``age`` field on the User
   class.

The result of this query would be a list of User objects where all
users are older than 20.

The SELECT clause allows to specify both class identification
variables that signal the hydration of a complete entity class or
just fields of the entity using the syntax ``u.name``. Combinations
of both are also allowed and it is possible to wrap both fields and
identification values into aggregation and DQL functions. Numerical
fields can be part of computations using mathematical operations.
See the sub-section on
`DQL Functions, Aggregates and Operations <#dqlfn>`_ on more
information.

Joins
~~~~~

A SELECT query can contain joins. There are 2 types of JOINs:
"Regular" Joins and "Fetch" Joins.

**Regular Joins**: Used to limit the results and/or compute
aggregate values.

**Fetch Joins**: In addition to the uses of regular joins: Used to
fetch related entities and include them in the hydrated result of a
query.

There is no special DQL keyword that distinguishes a regular join
from a fetch join. A join (be it an inner or outer join) becomes a
"fetch join" as soon as fields of the joined entity appear in the
SELECT part of the DQL query outside of an aggregate function.
Otherwise its a "regular join".

Example:

Regular join of the address:

.. code-block:: php

    <?php
    $query = $em->createQuery("SELECT u FROM User u JOIN u.address a WHERE a.city = 'Berlin'");
    $users = $query->getResult();

Fetch join of the address:

.. code-block:: php

    <?php
    $query = $em->createQuery("SELECT u, a FROM User u JOIN u.address a WHERE a.city = 'Berlin'");
    $users = $query->getResult();

When Doctrine hydrates a query with fetch-join it returns the class
in the FROM clause on the root level of the result array. In the
previous example an array of User instances is returned and the
address of each user is fetched and hydrated into the
``User#address`` variable. If you access the address Doctrine does
not need to lazy load the association with another query.

.. note::

    Doctrine allows you to walk all the associations between
    all the objects in your domain model. Objects that were not already
    loaded from the database are replaced with lazy load proxy
    instances. Non-loaded Collections are also replaced by lazy-load
    instances that fetch all the contained objects upon first access.
    However relying on the lazy-load mechanism leads to many small
    queries executed against the database, which can significantly
    affect the performance of your application. **Fetch Joins** are the
    solution to hydrate most or all of the entities that you need in a
    single SELECT query.


Named and Positional Parameters
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

DQL supports both named and positional parameters, however in
contrast to many SQL dialects positional parameters are specified
with numbers, for example "?1", "?2" and so on. Named parameters
are specified with ":name1", ":name2" and so on.

When referencing the parameters in ``Query#setParameter($param, $value)``
both named and positional parameters are used **without** their prefixes.

DQL SELECT Examples
~~~~~~~~~~~~~~~~~~~

This section contains a large set of DQL queries and some
explanations of what is happening. The actual result also depends
on the hydration mode.

Hydrate all User entities:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM MyProject\Model\User u');
    $users = $query->getResult(); // array of User objects

Retrieve the IDs of all CmsUsers:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u.id FROM CmsUser u');
    $ids = $query->getResult(); // array of CmsUser ids

Retrieve the IDs of all users that have written an article:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT DISTINCT u.id FROM CmsArticle a JOIN a.user u');
    $ids = $query->getResult(); // array of CmsUser ids

Retrieve all articles and sort them by the name of the articles
users instance:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT a FROM CmsArticle a JOIN a.user u ORDER BY u.name ASC');
    $articles = $query->getResult(); // array of CmsArticle objects

Retrieve the Username and Name of a CmsUser:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u.username, u.name FROM CmsUser u');
    $users = $query->getResults(); // array of CmsUser username and name values
    echo $users[0]['username'];

Retrieve a ForumUser and his single associated entity:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u, a FROM ForumUser u JOIN u.avatar a');
    $users = $query->getResult(); // array of ForumUser objects with the avatar association loaded
    echo get_class($users[0]->getAvatar());

Retrieve a CmsUser and fetch join all the phonenumbers he has:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u, p FROM CmsUser u JOIN u.phonenumbers p');
    $users = $query->getResult(); // array of CmsUser objects with the phonenumbers association loaded
    $phonenumbers = $users[0]->getPhonenumbers();

Hydrate a result in Ascending:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM ForumUser u ORDER BY u.id ASC');
    $users = $query->getResult(); // array of ForumUser objects

Or in Descending Order:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM ForumUser u ORDER BY u.id DESC');
    $users = $query->getResult(); // array of ForumUser objects

Using Aggregate Functions:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT COUNT(u.id) FROM Entities\User u');
    $count = $query->getSingleScalarResult();

With WHERE Clause and Positional Parameter:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM ForumUser u WHERE u.id = ?1');
    $query->setParameter(1, 321);
    $users = $query->getResult(); // array of ForumUser objects

With WHERE Clause and Named Parameter:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM ForumUser u WHERE u.username = :name');
    $query->setParameter('name', 'Bob');
    $users = $query->getResult(); // array of ForumUser objects

With Nested Conditions in WHERE Clause:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u from ForumUser u WHERE (u.username = :name OR u.username = :name2) AND u.id = :id');
    $query->setParameters(array(
        'name' => 'Bob',
        'name2' => 'Alice',
        'id' => 321,
    ));
    $users = $query->getResult(); // array of ForumUser objects

With COUNT DISTINCT:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT COUNT(DISTINCT u.name) FROM CmsUser');
    $users = $query->getResult(); // array of ForumUser objects

With Arithmetic Expression in WHERE clause:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM CmsUser u WHERE ((u.id + 5000) * u.id + 3) < 10000000');
    $users = $query->getResult(); // array of ForumUser objects

Using a LEFT JOIN to hydrate all user-ids and optionally associated
article-ids:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u.id, a.id as article_id FROM CmsUser u LEFT JOIN u.articles a');
    $results = $query->getResult(); // array of user ids and every article_id for each user

Restricting a JOIN clause by additional conditions:

.. code-block:: php

    <?php
    $query = $em->createQuery("SELECT u FROM CmsUser u LEFT JOIN u.articles a WITH a.topic LIKE '%foo%'");
    $users = $query->getResult();

Using several Fetch JOINs:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u, a, p, c FROM CmsUser u JOIN u.articles a JOIN u.phonenumbers p JOIN a.comments c');
    $users = $query->getResult();

BETWEEN in WHERE clause:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u.name FROM CmsUser u WHERE u.id BETWEEN ?1 AND ?2');
    $query->setParameter(1, 123);
    $query->setParameter(2, 321);
    $usernames = $query->getResult();

DQL Functions in WHERE clause:

.. code-block:: php

    <?php
    $query = $em->createQuery("SELECT u.name FROM CmsUser u WHERE TRIM(u.name) = 'someone'");
    $usernames = $query->getResult();

IN() Expression:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u.name FROM CmsUser u WHERE u.id IN(46)');
    $usernames = $query->getResult();
    
    $query = $em->createQuery('SELECT u FROM CmsUser u WHERE u.id IN (1, 2)');
    $users = $query->getResult();
    
    $query = $em->createQuery('SELECT u FROM CmsUser u WHERE u.id NOT IN (1)');
    $users = $query->getResult();

CONCAT() DQL Function:

.. code-block:: php

    <?php
    $query = $em->createQuery("SELECT u.id FROM CmsUser u WHERE CONCAT(u.name, 's') = ?1");
    $query->setParameter(1, 'Jess');
    $ids = $query->getResult();
    
    $query = $em->createQuery('SELECT CONCAT(u.id, u.name) FROM CmsUser u WHERE u.id = ?1');
    $query->setParameter(1, 321);
    $idUsernames = $query->getResult();

EXISTS in WHERE clause with correlated Subquery

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u.id FROM CmsUser u WHERE EXISTS (SELECT p.phonenumber FROM CmsPhonenumber p WHERE p.user = u.id)');
    $ids = $query->getResult();

Get all users who are members of $group.

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u.id FROM CmsUser u WHERE :groupId MEMBER OF u.groups');
    $query->setParameter('groupId', $group);
    $ids = $query->getResult();

Get all users that have more than 1 phonenumber

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM CmsUser u WHERE SIZE(u.phonenumbers) > 1');
    $users = $query->getResult();

Get all users that have no phonenumber

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM CmsUser u WHERE u.phonenumbers IS EMPTY');
    $users = $query->getResult();

Get all instances of a specific type, for use with inheritance
hierarchies:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM Doctrine\Tests\Models\Company\CompanyPerson u WHERE u INSTANCE OF Doctrine\Tests\Models\Company\CompanyEmployee');
    $query = $em->createQuery('SELECT u FROM Doctrine\Tests\Models\Company\CompanyPerson u WHERE u INSTANCE OF ?1');
    $query = $em->createQuery('SELECT u FROM Doctrine\Tests\Models\Company\CompanyPerson u WHERE u NOT INSTANCE OF ?1');

Partial Object Syntax
^^^^^^^^^^^^^^^^^^^^^

By default when you run a DQL query in Doctrine and select only a
subset of the fields for a given entity, you do not receive objects
back. Instead, you receive only arrays as a flat rectangular result
set, similar to how you would if you were just using SQL directly
and joining some data.

If you want to select partial objects you can use the ``partial``
DQL keyword:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT partial u.{id, username} FROM CmsUser u');
    $users = $query->getResult(); // array of partially loaded CmsUser objects

You use the partial syntax when joining as well:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT partial u.{id, username}, partial a.{id, name} FROM CmsUser u JOIN u.articles a');
    $users = $query->getResult(); // array of partially loaded CmsUser objects

Using INDEX BY
~~~~~~~~~~~~~~

The INDEX BY construct is nothing that directly translates into SQL
but that affects object and array hydration. After each FROM and
JOIN clause you specify by which field this class should be indexed
in the result. By default a result is incremented by numerical keys
starting with 0. However with INDEX BY you can specify any other
column to be the key of your result, it really only makes sense
with primary or unique fields though:

.. code-block:: sql

    SELECT u.id, u.status, upper(u.name) nameUpper FROM User u INDEX BY u.id
    JOIN u.phonenumbers p INDEX BY p.phonenumber

Returns an array of the following kind, indexed by both user-id
then phonenumber-id:

.. code-block:: php

    array
      0 =>
        array
          1 =>
            object(stdClass)[299]
              public '__CLASS__' => string 'Doctrine\Tests\Models\CMS\CmsUser' (length=33)
              public 'id' => int 1
              ..
          'nameUpper' => string 'ROMANB' (length=6)
      1 =>
        array
          2 =>
            object(stdClass)[298]
              public '__CLASS__' => string 'Doctrine\Tests\Models\CMS\CmsUser' (length=33)
              public 'id' => int 2
              ...
          'nameUpper' => string 'JWAGE' (length=5)

UPDATE queries
--------------

DQL not only allows to select your Entities using field names, you
can also execute bulk updates on a set of entities using an
DQL-UPDATE query. The Syntax of an UPDATE query works as expected,
as the following example shows:

.. code-block:: sql

    UPDATE MyProject\Model\User u SET u.password = 'new' WHERE u.id IN (1, 2, 3)

References to related entities are only possible in the WHERE
clause and using sub-selects.

.. warning::

    DQL UPDATE statements are ported directly into a
    Database UPDATE statement and therefore bypass any locking scheme, events
    and do not increment the version column. Entities that are already
    loaded into the persistence context will *NOT* be synced with the
    updated database state. It is recommended to call
    ``EntityManager#clear()`` and retrieve new instances of any
    affected entity.


DELETE queries
--------------

DELETE queries can also be specified using DQL and their syntax is
as simple as the UPDATE syntax:

.. code-block:: sql

    DELETE MyProject\Model\User u WHERE u.id = 4

The same restrictions apply for the reference of related entities.

.. warning::

    DQL DELETE statements are ported directly into a
    Database DELETE statement and therefore bypass any events and checks for the
    version column if they are not explicitly added to the WHERE clause
    of the query. Additionally Deletes of specifies entities are *NOT*
    cascaded to related entities even if specified in the metadata.


Functions, Operators, Aggregates
--------------------------------

DQL Functions
~~~~~~~~~~~~~

The following functions are supported in SELECT, WHERE and HAVING
clauses:


-  ABS(arithmetic\_expression)
-  CONCAT(str1, str2)
-  CURRENT\_DATE() - Return the current date
-  CURRENT\_TIME() - Returns the current time
-  CURRENT\_TIMESTAMP() - Returns a timestamp of the current date
   and time.
-  LENGTH(str) - Returns the length of the given string
-  LOCATE(needle, haystack [, offset]) - Locate the first
   occurrence of the substring in the string.
-  LOWER(str) - returns the string lowercased.
-  MOD(a, b) - Return a MOD b.
-  SIZE(collection) - Return the number of elements in the
   specified collection
-  SQRT(q) - Return the square-root of q.
-  SUBSTRING(str, start [, length]) - Return substring of given
   string.
-  TRIM([LEADING \| TRAILING \| BOTH] ['trchar' FROM] str) - Trim
   the string by the given trim char, defaults to whitespaces.
-  UPPER(str) - Return the upper-case of the given string.

Arithmetic operators
~~~~~~~~~~~~~~~~~~~~

You can do math in DQL using numeric values, for example:

.. code-block:: sql

    SELECT person.salary * 1.5 FROM CompanyPerson person WHERE person.salary < 100000

Aggregate Functions
~~~~~~~~~~~~~~~~~~~

The following aggregate functions are allowed in SELECT and GROUP
BY clauses: AVG, COUNT, MIN, MAX, SUM

Other Expressions
~~~~~~~~~~~~~~~~~

DQL offers a wide-range of additional expressions that are known
from SQL, here is a list of all the supported constructs:


-  ``ALL/ANY/SOME`` - Used in a WHERE clause followed by a
   sub-select this works like the equivalent constructs in SQL.
-  ``BETWEEN a AND b`` and ``NOT BETWEEN a AND b`` can be used to
   match ranges of arithmetic values.
-  ``IN (x1, x2, ...)`` and ``NOT IN (x1, x2, ..)`` can be used to
   match a set of given values.
-  ``LIKE ..`` and ``NOT LIKE ..`` match parts of a string or text
   using % as a wildcard.
-  ``IS NULL`` and ``IS NOT NULL`` to check for null values
-  ``EXISTS`` and ``NOT EXISTS`` in combination with a sub-select

Adding your own functions to the DQL language
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

By default DQL comes with functions that are part of a large basis
of underlying databases. However you will most likely choose a
database platform at the beginning of your project and most likely
never change it. For this cases you can easily extend the DQL
parser with own specialized platform functions.

You can register custom DQL functions in your ORM Configuration:

.. code-block:: php

    <?php
    $config = new \Doctrine\ORM\Configuration();
    $config->addCustomStringFunction($name, $class);
    $config->addCustomNumericFunction($name, $class);
    $config->addCustomDatetimeFunction($name, $class);
    
    $em = EntityManager::create($dbParams, $config);

The functions have to return either a string, numeric or datetime
value depending on the registered function type. As an example we
will add a MySQL specific FLOOR() functionality. All the given
classes have to implement the base class :

.. code-block:: php

    <?php
    namespace MyProject\Query\AST;
    
    use \Doctrine\ORM\Query\AST\Functions\FunctionsNode;
    
    class MysqlFloor extends FunctionNode
    {
        public $simpleArithmeticExpression;
    
        public function getSql(\Doctrine\ORM\Query\SqlWalker $sqlWalker)
        {
            return 'FLOOR(' . $sqlWalker->walkSimpleArithmeticExpression(
                $this->simpleArithmeticExpression
            ) . ')';
        }
    
        public function parse(\Doctrine\ORM\Query\Parser $parser)
        {
            $lexer = $parser->getLexer();
    
            $parser->match(Lexer::T_ABS);
            $parser->match(Lexer::T_OPEN_PARENTHESIS);
    
            $this->simpleArithmeticExpression = $parser->SimpleArithmeticExpression();
    
            $parser->match(Lexer::T_CLOSE_PARENTHESIS);
        }
    }

We will register the function by calling and can then use it:

.. code-block:: php

    <?php
    \Doctrine\ORM\Query\Parser::registerNumericFunction('FLOOR', 'MyProject\Query\MysqlFloor');
    $dql = "SELECT FLOOR(person.salary * 1.75) FROM CompanyPerson person";

Querying Inherited Classes
--------------------------

This section demonstrates how you can query inherited classes and
what type of results to expect.

Single Table
~~~~~~~~~~~~

`Single Table Inheritance <http://martinfowler.com/eaaCatalog/singleTableInheritance.html>`_
is an inheritance mapping strategy where all classes of a hierarchy
are mapped to a single database table. In order to distinguish
which row represents which type in the hierarchy a so-called
discriminator column is used.

First we need to setup an example set of entities to use. In this
scenario it is a generic Person and Employee example:

.. code-block:: php

    <?php
    namespace Entities;
    
    /**
     * @Entity
     * @InheritanceType("SINGLE_TABLE")
     * @DiscriminatorColumn(name="discr", type="string")
     * @DiscriminatorMap({"person" = "Person", "employee" = "Employee"})
     */
    class Person
    {
        /**
         * @Id @Column(type="integer")
         * @GeneratedValue
         */
        protected $id;
    
        /**
         * @Column(type="string", length=50)
         */
        protected $name;
    
        // ...
    }
    
    /**
     * @Entity
     */
    class Employee extends Person
    {
        /**
         * @Column(type="string", length=50)
         */
        private $department;
    
        // ...
    }

First notice that the generated SQL to create the tables for these
entities looks like the following:

.. code-block:: sql

    CREATE TABLE Person (
        id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
        name VARCHAR(50) NOT NULL,
        discr VARCHAR(255) NOT NULL,
        department VARCHAR(50) NOT NULL
    )

Now when persist a new ``Employee`` instance it will set the
discriminator value for us automatically:

.. code-block:: php

    <?php
    $employee = new \Entities\Employee();
    $employee->setName('test');
    $employee->setDepartment('testing');
    $em->persist($employee);
    $em->flush();

Now lets run a simple query to retrieve the ``Employee`` we just
created:

.. code-block:: sql

    SELECT e FROM Entities\Employee e WHERE e.name = 'test'

If we check the generated SQL you will notice it has some special
conditions added to ensure that we will only get back ``Employee``
entities:

.. code-block:: sql

    SELECT p0_.id AS id0, p0_.name AS name1, p0_.department AS department2,
           p0_.discr AS discr3 FROM Person p0_
    WHERE (p0_.name = ?) AND p0_.discr IN ('employee')

Class Table Inheritance
~~~~~~~~~~~~~~~~~~~~~~~

`Class Table Inheritance <http://martinfowler.com/eaaCatalog/classTableInheritance.html>`_
is an inheritance mapping strategy where each class in a hierarchy
is mapped to several tables: its own table and the tables of all
parent classes. The table of a child class is linked to the table
of a parent class through a foreign key constraint. Doctrine 2
implements this strategy through the use of a discriminator column
in the topmost table of the hierarchy because this is the easiest
way to achieve polymorphic queries with Class Table Inheritance.

The example for class table inheritance is the same as single
table, you just need to change the inheritance type from
``SINGLE_TABLE`` to ``JOINED``:

.. code-block:: php

    <?php
    /**
     * @Entity
     * @InheritanceType("JOINED")
     * @DiscriminatorColumn(name="discr", type="string")
     * @DiscriminatorMap({"person" = "Person", "employee" = "Employee"})
     */
    class Person
    {
        // ...
    }

Now take a look at the SQL which is generated to create the table,
you'll notice some differences:

.. code-block:: sql

    CREATE TABLE Person (
        id INT AUTO_INCREMENT NOT NULL,
        name VARCHAR(50) NOT NULL,
        discr VARCHAR(255) NOT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;
    CREATE TABLE Employee (
        id INT NOT NULL,
        department VARCHAR(50) NOT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;
    ALTER TABLE Employee ADD FOREIGN KEY (id) REFERENCES Person(id) ON DELETE CASCADE


-  The data is split between two tables
-  A foreign key exists between the two tables

Now if were to insert the same ``Employee`` as we did in the
``SINGLE_TABLE`` example and run the same example query it will
generate different SQL joining the ``Person`` information
automatically for you:

.. code-block:: sql

    SELECT p0_.id AS id0, p0_.name AS name1, e1_.department AS department2,
           p0_.discr AS discr3
    FROM Employee e1_ INNER JOIN Person p0_ ON e1_.id = p0_.id
    WHERE p0_.name = ?


The Query class
---------------

An instance of the ``Doctrine\ORM\Query`` class represents a DQL
query. You create a Query instance be calling
``EntityManager#createQuery($dql)``, passing the DQL query string.
Alternatively you can create an empty ``Query`` instance and invoke
``Query#setDql($dql)`` afterwards. Here are some examples:

.. code-block:: php

    <?php
    // $em instanceof EntityManager
    
    // example1: passing a DQL string
    $q = $em->createQuery('select u from MyProject\Model\User u');
    
    // example2: using setDql
    $q = $em->createQuery();
    $q->setDql('select u from MyProject\Model\User u');

Query Result Formats
~~~~~~~~~~~~~~~~~~~~

The format in which the result of a DQL SELECT query is returned
can be influenced by a so-called ``hydration mode``. A hydration
mode specifies a particular way in which an SQL result set is
transformed. Each hydration mode has its own dedicated method on
the Query class. Here they are:


-  ``Query#getResult()``: Retrieves a collection of objects. The
   result is either a plain collection of objects (pure) or an array
   where the objects are nested in the result rows (mixed).
-  ``Query#getSingleResult()``: Retrieves a single object. If the
   result contains more than one object, an exception is thrown. The
   pure/mixed distinction does not apply.
-  ``Query#getArrayResult()``: Retrieves an array graph (a nested
   array) that is largely interchangeable with the object graph
   generated by ``Query#getResult()`` for read-only purposes.

    .. note::

        An array graph can differ from the corresponding object
        graph in certain scenarios due to the difference of the identity
        semantics between arrays and objects.



-  ``Query#getScalarResult()``: Retrieves a flat/rectangular result
   set of scalar values that can contain duplicate data. The
   pure/mixed distinction does not apply.
-  ``Query#getSingleScalarResult()``: Retrieves a single scalar
   value from the result returned by the dbms. If the result contains
   more than a single scalar value, an exception is thrown. The
   pure/mixed distinction does not apply.

Instead of using these methods, you can alternatively use the
general-purpose method
``Query#execute(array $params = array(), $hydrationMode = Query::HYDRATE_OBJECT)``.
Using this method you can directly supply the hydration mode as the
second parameter via one of the Query constants. In fact, the
methods mentioned earlier are just convenient shortcuts for the
execute method. For example, the method ``Query#getResult()``
internally invokes execute, passing in ``Query::HYDRATE_OBJECT`` as
the hydration mode.

The use of the methods mentioned earlier is generally preferred as
it leads to more concise code.

Pure and Mixed Results
~~~~~~~~~~~~~~~~~~~~~~

The nature of a result returned by a DQL SELECT query retrieved
through ``Query#getResult()`` or ``Query#getArrayResult()`` can be
of 2 forms: **pure** and **mixed**. In the previous simple
examples, you already saw a "pure" query result, with only objects.
By default, the result type is **pure** but
**as soon as scalar values, such as aggregate values or other scalar values that do not belong to an entity, appear in the SELECT part of the DQL query, the result becomes mixed**.
A mixed result has a different structure than a pure result in
order to accommodate for the scalar values.

A pure result usually looks like this:

.. code-block:: php

    array
        [0] => Object
        [1] => Object
        [2] => Object
        ...

A mixed result on the other hand has the following general
structure:

.. code-block:: php

    array
        [0]
            [0] => Object
            [1] => "some scalar string"
            ['count'] => 42
            // ... more scalar values, either indexed numerically or with a name
        [1]
            [0] => Object
            [1] => "some scalar string"
            ['count'] => 42
            // ... more scalar values, either indexed numerically or with a name

To better understand mixed results, consider the following DQL
query:

.. code-block:: sql

    SELECT u, UPPER(u.name) nameUpper FROM MyProject\Model\User u

This query makes use of the ``UPPER`` DQL function that returns a
scalar value and because there is now a scalar value in the SELECT
clause, we get a mixed result.

Here is how the result could look like:

.. code-block:: php

    array
        array
            [0] => User (Object)
            ['nameUpper'] => "ROMAN"
        array
            [0] => User (Object)
            ['nameUpper'] => "JONATHAN"
        ...

And here is how you would access it in PHP code:

.. code-block:: php

    <?php
    foreach ($results as $row) {
        echo "Name: " . $row[0]->getName();
        echo "Name UPPER: " . $row['nameUpper'];
    }

You may have observed that in a mixed result, the object always
ends up on index 0 of a result row.

Hydration Modes
~~~~~~~~~~~~~~~

Each of the Hydration Modes makes assumptions about how the result
is returned to user land. You should know about all the details to
make best use of the different result formats:

The constants for the different hydration modes are:


-  Query::HYDRATE\_OBJECT
-  Query::HYDRATE\_ARRAY
-  Query::HYDRATE\_SCALAR
-  Query::HYDRATE\_SINGLE\_SCALAR

Object Hydration
^^^^^^^^^^^^^^^^

Object hydration hydrates the result set into the object graph:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM CmsUser u');
    $users = $query->getResult(Query::HYDRATE_OBJECT);

Array Hydration
^^^^^^^^^^^^^^^

You can run the same query with array hydration and the result set
is hydrated into an array that represents the object graph:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM CmsUser u');
    $users = $query->getResult(Query::HYDRATE_ARRAY);

You can use the ``getArrayResult()`` shortcut as well:

.. code-block:: php

    <?php
    $users = $query->getArrayResult();

Scalar Hydration
^^^^^^^^^^^^^^^^

If you want to return a flat rectangular result set instead of an
object graph you can use scalar hydration:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM CmsUser u');
    $users = $query->getResult(Query::HYDRATE_SCALAR);
    echo $users[0]['u_id'];

The following assumptions are made about selected fields using
Scalar Hydration:


1. Fields from classes are prefixed by the DQL alias in the result.
   A query of the kind 'SELECT u.name ..' returns a key 'u\_name' in
   the result rows.

Single Scalar Hydration
^^^^^^^^^^^^^^^^^^^^^^^

If you a query which returns just a single scalar value you can use
single scalar hydration:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT COUNT(a.id) FROM CmsUser u LEFT JOIN u.articles a WHERE u.username = ?1 GROUP BY u.id');
    $query->setParameter(1, 'jwage');
    $numArticles = $query->getResult(Query::HYDRATE_SINGLE_SCALAR);

You can use the ``getSingleScalarResult()`` shortcut as well:

.. code-block:: php

    <?php
    $numArticles = $query->getSingleScalarResult();

Custom Hydration Modes
^^^^^^^^^^^^^^^^^^^^^^

You can easily add your own custom hydration modes by first
creating a class which extends ``AbstractHydrator``:

.. code-block:: php

    <?php
    namespace MyProject\Hydrators;
    
    use Doctrine\ORM\Internal\Hydration\AbstractHydrator;
    
    class CustomHydrator extends AbstractHydrator
    {
        protected function _hydrateAll()
        {
            return $this->_stmt->fetchAll(PDO::FETCH_ASSOC);
        }
    }

Next you just need to add the class to the ORM configuration:

.. code-block:: php

    <?php
    $em->getConfiguration()->addCustomHydrationMode('CustomHydrator', 'MyProject\Hydrators\CustomHydrator');

Now the hydrator is ready to be used in your queries:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM CmsUser u');
    $results = $query->getResult('CustomHydrator');

Iterating Large Result Sets
~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are situations when a query you want to execute returns a
very large result-set that needs to be processed. All the
previously described hydration modes completely load a result-set
into memory which might not be feasible with large result sets. See
the `Batch Processing <batch-processing>`_ section on details how
to iterate large result sets.

Functions
~~~~~~~~~

The following methods exist on the ``AbstractQuery`` which both
``Query`` and ``NativeQuery`` extend from.

Parameters
^^^^^^^^^^

Prepared Statements that use numerical or named wildcards require
additional parameters to be executable against the database. To
pass parameters to the query the following methods can be used:


-  ``AbstractQuery::setParameter($param, $value)`` - Set the
   numerical or named wildcard to the given value.
-  ``AbstractQuery::setParameters(array $params)`` - Set an array
   of parameter key-value pairs.
-  ``AbstractQuery::getParameter($param)``
-  ``AbstractQuery::getParameters()``

Both named and positional parameters are passed to these methods without their ? or : prefix.

Cache related API
^^^^^^^^^^^^^^^^^

You can cache query results based either on all variables that
define the result (SQL, Hydration Mode, Parameters and Hints) or on
user-defined cache keys. However by default query results are not
cached at all. You have to enable the result cache on a per query
basis. The following example shows a complete workflow using the
Result Cache API:

.. code-block:: php

    <?php
    $query = $em->createQuery('SELECT u FROM MyProject\Model\User u WHERE u.id = ?1');
    $query->setParameter(1, 12);
    
    $query->setResultCacheDriver(new ApcCache());
    
    $query->useResultCache(true)
          ->setResultCacheLifeTime($seconds = 3600);
    
    $result = $query->getResult(); // cache miss
    
    $query->expireResultCache(true);
    $result = $query->getResult(); // forced expire, cache miss
    
    $query->setResultCacheId('my_query_result');
    $result = $query->getResult(); // saved in given result cache id.
    
    // or call useResultCache() with all parameters:
    $query->useResultCache(true, $seconds = 3600, 'my_query_result');
    $result = $query->getResult(); // cache hit!

    **TIP!** You can set the Result Cache Driver globally on the
    ``Doctrine\ORM\Configuration`` instance so that it is passed to
    every ``Query`` and ``NativeQuery`` instance.


Query Hints
^^^^^^^^^^^

You can pass hints to the query parser and hydrators by using the
``AbstractQuery::setHint($name, $value)`` method. Currently there
exist mostly internal query hints that are not be consumed in
userland. However the following few hints are to be used in
userland:


-  Query::HINT\_FORCE\_PARTIAL\_LOAD - Allows to hydrate objects
   although not all their columns are fetched. This query hint can be
   used to handle memory consumption problems with large result-sets
   that contain char or binary data. Doctrine has no way of implicitly
   reloading this data. Partially loaded objects have to be passed to
   ``EntityManager::refresh()`` if they are to be reloaded fully from
   the database.
-  Query::HINT\_REFRESH - This query is used internally by
   ``EntityManager::refresh()`` and can be used in userland as well.
   If you specify this hint and a query returns the data for an entity
   that is already managed by the UnitOfWork, the fields of the
   existing entity will be refreshed. In normal operation a result-set
   that loads data of an already existing entity is discarded in favor
   of the already existing entity.
-  Query::HINT\_CUSTOM\_TREE\_WALKERS - An array of additional
   ``Doctrine\ORM\Query\TreeWalker`` instances that are attached to
   the DQL query parsing process.

Query Cache (DQL Query Only)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Parsing a DQL query and converting it into an SQL query against the
underlying database platform obviously has some overhead in
contrast to directly executing Native SQL queries. That is why
there is a dedicated Query Cache for caching the DQL parser
results. In combination with the use of wildcards you can reduce
the number of parsed queries in production to zero.

The Query Cache Driver is passed from the
``Doctrine\ORM\Configuration`` instance to each
``Doctrine\ORM\Query`` instance by default and is also enabled by
default. This also means you don't regularly need to fiddle with
the parameters of the Query Cache, however if you do there are
several methods to interact with it:


-  ``Query::setQueryCacheDriver($driver)`` - Allows to set a Cache
   instance
-  ``Query::setQueryCacheLifeTime($seconds = 3600)`` - Set lifetime
   of the query caching.
-  ``Query::expireQueryCache($bool)`` - Enforce the expiring of the
   query cache if set to true.
-  ``Query::getExpireQueryCache()``
-  ``Query::getQueryCacheDriver()``
-  ``Query::getQueryCacheLifeTime()``

First and Max Result Items (DQL Query Only)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You can limit the number of results returned from a DQL query as
well as specify the starting offset, Doctrine then uses a strategy
of manipulating the select query to return only the requested
number of results:


-  ``Query::setMaxResults($maxResults)``
-  ``Query::setFirstResult($offset)``

.. note::

    If your query contains a fetch-joined collection
    specifying the result limit methods are not working as you would
    expect. Set Max Results restricts the number of database result
    rows, however in the case of fetch-joined collections one root
    entity might appear in many rows, effectively hydrating less than
    the specified number of results.


EBNF
----

The following context-free grammar, written in an EBNF variant,
describes the Doctrine Query Language. You can consult this grammar
whenever you are unsure about what is possible with DQL or what the
correct syntax for a particular query should be.

Document syntax:
~~~~~~~~~~~~~~~~


-  non-terminals begin with an upper case character
-  terminals begin with a lower case character
-  parentheses (...) are used for grouping
-  square brackets [...] are used for defining an optional part,
   e.g. zero or one time
-  curly brackets {...} are used for repetition, e.g. zero or more
   times
-  double quotation marks "..." define a terminal string a vertical
   bar \| represents an alternative

Terminals
~~~~~~~~~


-  identifier (name, email, ...)
-  string ('foo', 'bar''s house', '%ninja%', ...)
-  char ('/', '\\', ' ', ...)
-  integer (-1, 0, 1, 34, ...)
-  float (-0.23, 0.007, 1.245342E+8, ...)
-  boolean (false, true)

Query Language
~~~~~~~~~~~~~~

.. code-block:: php

    QueryLanguage ::= SelectStatement | UpdateStatement | DeleteStatement

Statements
~~~~~~~~~~

.. code-block:: php

    SelectStatement ::= SelectClause FromClause [WhereClause] [GroupByClause] [HavingClause] [OrderByClause]
    UpdateStatement ::= UpdateClause [WhereClause]
    DeleteStatement ::= DeleteClause [WhereClause]

Identifiers
~~~~~~~~~~~

.. code-block:: php

    /* Alias Identification usage (the "u" of "u.name") */
    IdentificationVariable ::= identifier
    
    /* Alias Identification declaration (the "u" of "FROM User u") */
    AliasIdentificationVariable :: = identifier
    
    /* identifier that must be a class name (the "User" of "FROM User u") */
    AbstractSchemaName ::= identifier
    
    /* identifier that must be a field (the "name" of "u.name") */
    /* This is responsible to know if the field exists in Object, no matter if it's a relation or a simple field */
    FieldIdentificationVariable ::= identifier
    
    /* identifier that must be a collection-valued association field (to-many) (the "Phonenumbers" of "u.Phonenumbers") */
    CollectionValuedAssociationField ::= FieldIdentificationVariable
    
    /* identifier that must be a single-valued association field (to-one) (the "Group" of "u.Group") */
    SingleValuedAssociationField ::= FieldIdentificationVariable
    
    /* identifier that must be an embedded class state field (for the future) */
    EmbeddedClassStateField ::= FieldIdentificationVariable
    
    /* identifier that must be a simple state field (name, email, ...) (the "name" of "u.name") */
    /* The difference between this and FieldIdentificationVariable is only semantical, because it points to a single field (not mapping to a relation) */
    SimpleStateField ::= FieldIdentificationVariable
    
    /* Alias ResultVariable declaration (the "total" of "COUNT(*) AS total") */
    AliasResultVariable = identifier
    
    /* ResultVariable identifier usage of mapped field aliases (the "total" of "COUNT(*) AS total") */
    ResultVariable = identifier

Path Expressions
~~~~~~~~~~~~~~~~

.. code-block:: php

    /* "u.Group" or "u.Phonenumbers" declarations */
    JoinAssociationPathExpression             ::= IdentificationVariable "." (CollectionValuedAssociationField | SingleValuedAssociationField)
    
    /* "u.Group" or "u.Phonenumbers" usages */
    AssociationPathExpression                 ::= CollectionValuedPathExpression | SingleValuedAssociationPathExpression
    
    /* "u.name" or "u.Group" */
    SingleValuedPathExpression                ::= StateFieldPathExpression | SingleValuedAssociationPathExpression
    
    /* "u.name" or "u.Group.name" */
    StateFieldPathExpression                  ::= IdentificationVariable "." StateField | SingleValuedAssociationPathExpression "." StateField
    
    /* "u.Group" */
    SingleValuedAssociationPathExpression     ::= IdentificationVariable "." SingleValuedAssociationField

    /* "u.Group.Permissions" */
    CollectionValuedPathExpression            ::= IdentificationVariable "." {SingleValuedAssociationField "."}* CollectionValuedAssociationField
    
    /* "name" */
    StateField                                ::= {EmbeddedClassStateField "."}* SimpleStateField
    
    /* "u.name" or "u.address.zip" (address = EmbeddedClassStateField) */
    SimpleStateFieldPathExpression            ::= IdentificationVariable "." StateField

Clauses
~~~~~~~

.. code-block:: php

    SelectClause        ::= "SELECT" ["DISTINCT"] SelectExpression {"," SelectExpression}*
    SimpleSelectClause  ::= "SELECT" ["DISTINCT"] SimpleSelectExpression
    UpdateClause        ::= "UPDATE" AbstractSchemaName ["AS"] AliasIdentificationVariable "SET" UpdateItem {"," UpdateItem}*
    DeleteClause        ::= "DELETE" ["FROM"] AbstractSchemaName ["AS"] AliasIdentificationVariable
    FromClause          ::= "FROM" IdentificationVariableDeclaration {"," IdentificationVariableDeclaration}*
    SubselectFromClause ::= "FROM" SubselectIdentificationVariableDeclaration {"," SubselectIdentificationVariableDeclaration}*
    WhereClause         ::= "WHERE" ConditionalExpression
    HavingClause        ::= "HAVING" ConditionalExpression
    GroupByClause       ::= "GROUP" "BY" GroupByItem {"," GroupByItem}*
    OrderByClause       ::= "ORDER" "BY" OrderByItem {"," OrderByItem}*
    Subselect           ::= SimpleSelectClause SubselectFromClause [WhereClause] [GroupByClause] [HavingClause] [OrderByClause]

Items
~~~~~

.. code-block:: php

    UpdateItem  ::= IdentificationVariable "." (StateField | SingleValuedAssociationField) "=" NewValue
    OrderByItem ::= (ResultVariable | StateFieldPathExpression) ["ASC" | "DESC"]
    GroupByItem ::= IdentificationVariable | SingleValuedPathExpression
    NewValue    ::= ScalarExpression | SimpleEntityExpression | "NULL"

From, Join and Index by
~~~~~~~~~~~~~~~~~~~~~~~

.. code-block:: php

    IdentificationVariableDeclaration          ::= RangeVariableDeclaration [IndexBy] {JoinVariableDeclaration}*
    SubselectIdentificationVariableDeclaration ::= IdentificationVariableDeclaration | (AssociationPathExpression ["AS"] AliasIdentificationVariable)
    JoinVariableDeclaration                    ::= Join [IndexBy]
    RangeVariableDeclaration                   ::= AbstractSchemaName ["AS"] AliasIdentificationVariable
    Join                                       ::= ["LEFT" ["OUTER"] | "INNER"] "JOIN" JoinAssociationPathExpression
                                                   ["AS"] AliasIdentificationVariable ["WITH" ConditionalExpression]
    IndexBy                                    ::= "INDEX" "BY" SimpleStateFieldPathExpression

Select Expressions
~~~~~~~~~~~~~~~~~~

.. code-block:: php

    SelectExpression       ::= IdentificationVariable | PartialObjectExpression | (AggregateExpression | "(" Subselect ")"  | FunctionDeclaration | ScalarExpression) [["AS"] AliasResultVariable]
    SimpleSelectExpression ::= ScalarExpression | IdentificationVariable |
                               (AggregateExpression [["AS"] AliasResultVariable])
    PartialObjectExpression ::= "PARTIAL" IdentificationVariable "." PartialFieldSet
    PartialFieldSet ::= "{" SimpleStateField {"," SimpleStateField}* "}"

Conditional Expressions
~~~~~~~~~~~~~~~~~~~~~~~

.. code-block:: php

    ConditionalExpression       ::= ConditionalTerm {"OR" ConditionalTerm}*
    ConditionalTerm             ::= ConditionalFactor {"AND" ConditionalFactor}*
    ConditionalFactor           ::= ["NOT"] ConditionalPrimary
    ConditionalPrimary          ::= SimpleConditionalExpression | "(" ConditionalExpression ")"
    SimpleConditionalExpression ::= ComparisonExpression | BetweenExpression | LikeExpression |
                                    InExpression | NullComparisonExpression | ExistsExpression |
                                    EmptyCollectionComparisonExpression | CollectionMemberExpression

Collection Expressions
~~~~~~~~~~~~~~~~~~~~~~

.. code-block:: php

    EmptyCollectionComparisonExpression ::= CollectionValuedPathExpression "IS" ["NOT"] "EMPTY"
    CollectionMemberExpression          ::= EntityExpression ["NOT"] "MEMBER" ["OF"] CollectionValuedPathExpression

Literal Values
~~~~~~~~~~~~~~

.. code-block:: php

    Literal     ::= string | char | integer | float | boolean
    InParameter ::= Literal | InputParameter

Input Parameter
~~~~~~~~~~~~~~~

.. code-block:: php

    InputParameter      ::= PositionalParameter | NamedParameter
    PositionalParameter ::= "?" integer
    NamedParameter      ::= ":" string

Arithmetic Expressions
~~~~~~~~~~~~~~~~~~~~~~

.. code-block:: php

    ArithmeticExpression       ::= SimpleArithmeticExpression | "(" Subselect ")"
    SimpleArithmeticExpression ::= ArithmeticTerm {("+" | "-") ArithmeticTerm}*
    ArithmeticTerm             ::= ArithmeticFactor {("*" | "/") ArithmeticFactor}*
    ArithmeticFactor           ::= [("+" | "-")] ArithmeticPrimary
    ArithmeticPrimary          ::= SingleValuedPathExpression | Literal | "(" SimpleArithmeticExpression ")"
                                   | FunctionsReturningNumerics | AggregateExpression | FunctionsReturningStrings
                                   | FunctionsReturningDatetime | IdentificationVariable | InputParameter

Scalar and Type Expressions
~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. code-block:: php

    ScalarExpression       ::= SimpleArithmeticExpression | StringPrimary | DateTimePrimary | StateFieldPathExpression
                               BooleanPrimary | EntityTypeExpression
    StringExpression       ::= StringPrimary | "(" Subselect ")"
    StringPrimary          ::= StateFieldPathExpression | string | InputParameter | FunctionsReturningStrings | AggregateExpression
    BooleanExpression      ::= BooleanPrimary | "(" Subselect ")"
    BooleanPrimary         ::= StateFieldPathExpression | boolean | InputParameter
    EntityExpression       ::= SingleValuedAssociationPathExpression | SimpleEntityExpression
    SimpleEntityExpression ::= IdentificationVariable | InputParameter
    DatetimeExpression     ::= DatetimePrimary | "(" Subselect ")"
    DatetimePrimary        ::= StateFieldPathExpression | InputParameter | FunctionsReturningDatetime | AggregateExpression

.. note::

    Parts of CASE expressions are not yet implemented.

Aggregate Expressions
~~~~~~~~~~~~~~~~~~~~~

.. code-block:: php

    AggregateExpression ::= ("AVG" | "MAX" | "MIN" | "SUM") "(" ["DISTINCT"] StateFieldPathExpression ")" |
                            "COUNT" "(" ["DISTINCT"] (IdentificationVariable | SingleValuedPathExpression) ")"

Other Expressions
~~~~~~~~~~~~~~~~~

QUANTIFIED/BETWEEN/COMPARISON/LIKE/NULL/EXISTS

.. code-block:: php

    QuantifiedExpression     ::= ("ALL" | "ANY" | "SOME") "(" Subselect ")"
    BetweenExpression        ::= ArithmeticExpression ["NOT"] "BETWEEN" ArithmeticExpression "AND" ArithmeticExpression
    ComparisonExpression     ::= ArithmeticExpression ComparisonOperator ( QuantifiedExpression | ArithmeticExpression )
    InExpression             ::= StateFieldPathExpression ["NOT"] "IN" "(" (InParameter {"," InParameter}* | Subselect) ")"
    LikeExpression           ::= StringExpression ["NOT"] "LIKE" string ["ESCAPE" char]
    NullComparisonExpression ::= (SingleValuedPathExpression | InputParameter) "IS" ["NOT"] "NULL"
    ExistsExpression         ::= ["NOT"] "EXISTS" "(" Subselect ")"
    ComparisonOperator       ::= "=" | "<" | "<=" | "<>" | ">" | ">=" | "!="

Functions
~~~~~~~~~

.. code-block:: php

    FunctionDeclaration ::= FunctionsReturningStrings | FunctionsReturningNumerics | FunctionsReturningDateTime
    
    FunctionsReturningNumerics ::=
            "LENGTH" "(" StringPrimary ")" |
            "LOCATE" "(" StringPrimary "," StringPrimary ["," SimpleArithmeticExpression]")" |
            "ABS" "(" SimpleArithmeticExpression ")" | "SQRT" "(" SimpleArithmeticExpression ")" |
            "MOD" "(" SimpleArithmeticExpression "," SimpleArithmeticExpression ")" |
            "SIZE" "(" CollectionValuedPathExpression ")"
    
    FunctionsReturningDateTime ::= "CURRENT_DATE" | "CURRENT_TIME" | "CURRENT_TIMESTAMP"
    
    FunctionsReturningStrings ::=
            "CONCAT" "(" StringPrimary "," StringPrimary ")" |
            "SUBSTRING" "(" StringPrimary "," SimpleArithmeticExpression "," SimpleArithmeticExpression ")" |
            "TRIM" "(" [["LEADING" | "TRAILING" | "BOTH"] [char] "FROM"] StringPrimary ")" |
            "LOWER" "(" StringPrimary ")" |
            "UPPER" "(" StringPrimary ")"