realm-query-language.txt

.. _realm-query-language:
.. _rql:

====================
Realm Query Language
====================

.. contents:: On this page
   :local:
   :backlinks: none
   :depth: 3
   :class: singlecol

Realm Query Language (RQL) is a string-based query language to constrain
searches when retrieving objects from a realm. SDK-specific methods pass queries
to the Realm query engine, which retrieves matching objects from the realm.
Realm Query Language syntax is based on `NSPredicate
<https://developer.apple.com/documentation/foundation/nspredicate>`__.

Queries evaluate a predicate for every object in the collection being queried.
If the predicate resolves to ``true``, the results collection includes the object.

You can use Realm Query Language in most Realm SDKs with your SDK's filter
or query methods. The Swift SDK is the exception, as it uses the
:ref:`NSPredicate query API <ios-nspredicate-query>`.
Some SDKs also support idiomatic APIs for querying realms in their language.

Query with Realm SDKs
---------------------

For further reading on SDK-specific methods for querying realms, select the tab
below for your SDK.

.. tabs::

   .. tab:: Java SDK
      :tabid: java

      - :ref:`Query Engine - Java SDK <java-client-query-engine>`
      - :ref:`Query with Realm Query Language - Java SDK <java-filter-with-realm-query-language>`

   .. tab:: Swift SDK
      :tabid: swift

      - :ref:`Filter Data - Swift SDK <ios-client-query-engine>`

      .. note:: Swift SDK does not support Realm Query Language

         The Swift SDK does not support querying with Realm Query Language.
         You can instead use NSPredicate to query Realm.

   .. tab:: .NET SDK
      :tabid: dotnet

      - :ref:`Query Engine - .NET SDK <dotnet-client-query-engine>`

   .. tab:: Node.JS SDK
      :tabid: node

      - :ref:`Query Engine - Node.Js SDK <node-client-query-engine>`
      - :ref:`Filter Queries - Node.Js SDK <node-filter-queries>`

   .. tab:: React Native SDK
      :tabid: react-native

      - :ref:`Query Engine - React Native SDK <react-native-client-query-engine>`
      - :ref:`Filter Queries - React Native SDK <react-native-filter-queries>`

   .. tab:: Kotlin SDK
      :tabid: kotlin

      - :ref:`Filter Data - Kotlin SDK <kotlin-filter-data>`

   .. tab:: Flutter SDK
      :tabid: flutter

      - :ref:`Filter & Sort Results - Flutter SDK <flutter-filter-results>`


You can also use Realm Query Language to browse for data in
:ref:`Realm Studio <realm-studio>`. Realm Studio is a visual tool
to view, edit, and design Realm Database files.

Examples on This Page
---------------------

Many of the examples in this page use a simple data set for a task list app.
The two Realm object types are ``Project`` and ``Task``.

- A ``Task`` has a name, assignee's name, and completed flag.
  There is also an arbitrary number for priority (higher is more important)
  and a count of minutes spent working on it.
- A ``Project`` has zero or more ``Tasks`` and an optional quota
  for minimum number of tasks expected to be completed.

See the schema for these two classes, ``Project`` and ``Task``, below:

.. tabs::

   .. tab:: Java SDK
      :tabid: java

      .. tabs-realm-languages::

         .. tab::
             :tabid: java

             .. code-block:: java

               public class Task extends RealmObject {
                 ObjectId id = new ObjectId();
                 String name;
                 Boolean isComplete = false;
                 String assignee;
                 Integer priority = 0;
                 Integer progressMinutes = 0;
                 @LinkingObjects("tasks")
                 final RealmResults<Project> projects = null;
               } 
               public class Project extends RealmObject {
                 ObjectId id = new ObjectId();
                 String name;
                 RealmList<Task> tasks;
                 Integer quota = null;
               }

         .. tab::
            :tabid: kotlin

            .. code-block:: kotlin

               open class Task(): RealmObject() {
                 var id: ObjectId = new ObjectId()
                 lateinit var name: String
                 var isComplete: Boolean = false
                 var assignee: String? = null
                 var priority: Int = 0
                 var progressMinutes: Int = 0
               }
               
               open class Project(): RealmObject() {
                 var id: ObjectId = new ObjectId()
                 lateinit var name: String
                 lateinit var tasks: RealmList<Task>
                 var quota: Int? = null
               }

   .. tab:: Swift SDK
      :tabid: swift

      .. code-block:: swift

         class Task {
             @Persisted(primaryKey: true) var id: ObjectId
             @Persisted var name: string
             @Persisted var isComplete: bool
             @Persisted var assignee: string?
             @Persisted var priority: int
             @Persisted var progressMinutes: int
             @Persisted(originProperty: "tasks") var projects: LinkingObjects<Project>
         }

         class Project {
             @Persisted(primaryKey: true) var id: ObjectId
             @Persisted var name: string
             @Persisted var tasks: List<Task>
             @Persisted var quota: int?
         }

   .. tab:: .NET SDK
      :tabid: dotnet

      .. literalinclude:: /examples/generated/dotnet/RqlSchemaExamples.snippet.rql-schema-examples.cs
         :language: csharp

   .. tab:: Node.Js SDK
      :tabid: node

      .. literalinclude:: /examples/generated/node/rql-data-models.snippet.rql-data-models.js
         :language: javascript

   .. tab:: React Native SDK
      :tabid: react-native

      .. literalinclude:: /examples/generated/node/rql-data-models.snippet.rql-data-models.js
         :language: javascript

   .. tab:: Kotlin SDK
      :tabid: kotlin

      .. code-block:: kotlin

         class Task(): RealmObject {
           var id: Long = 0 // Kotlin SDK does not yet support ObjectId
           lateinit var name: String
           var isComplete: Boolean = false
           var assignee: String? = null
           var priority: Int = 0
           var progressMinutes: Int = 0
           @LinkingObjects("tasks")
           val projects: RealmResults<Project>? = null
         }

         class Project(): RealmObject {
           var id: Long = 0 // Kotlin SDK does not yet support ObjectId
           lateinit var name: String
           lateinit var tasks: Array<Task>
           var quota: Int? = null
         }

   .. tab:: Flutter SDK
      :tabid: Flutter

      .. literalinclude:: /examples/generated/flutter/task_project_models_test.snippet.task-project-models.dart
         :language: dart

Expressions
-----------

Filters consist of **expressions** in a predicate. An expression consists of
one of the following:

- The name of a property of the object currently being evaluated.
- An operator and up to two argument expression(s). For example, in the
  expression ``A + B``, the entirety of ``A + B`` is an expression, but ``A``
  and ``B`` are also argument expressions to the operator ``+``.
- A value, such as a string (``'hello'``) or a number (``5``).

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.predicate.js
   :language: javascript

.. _rql-parameterized-queries:

Parameterized Queries
---------------------

Create parameterized queries to interpolate variables into prepared
Realm Query Language statements. The syntax for interpolated variables is
``$<int>``, starting at ``0``. Pass the positional arguments as
additional arguments to Realm SDK methods that use Realm Query Language.

Include just one parameter with ``$0``.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.predicate.js
   :language: js

Include multiple parameters with ascending integers starting at ``$0``.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.multiple-predicate.js
   :language: js

.. _rql-dot-notation:

Dot Notation
------------

When referring to an object property, you can use **dot notation** to refer
to child properties of that object. You can even refer to the properties of
embedded objects and relationships with dot notation.

For example, consider a query on an object with a ``workplace`` property that
refers to a Workplace object. The Workplace object has an embedded object
property, ``address``. You can chain dot notations to refer to the zipcode
property of that address:

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.deep-dot-notation.js
   :language: js

.. _rql-nil-type:

Nil Type
--------

Realm Query Language include the ``nil`` type to represent a null pointer.
You can either reference ``nil`` directly in your queries or with a parameterized query.
If you're using a parameterized query, each SDK maps its respective null pointer
to ``nil``.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.nil-type.js
   :language: js

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.nil-type-parameterized-query.js
   :language: js

.. _rql-operators:
.. _rql-comparison-operators:

Comparison Operators
--------------------

The most straightforward operation in a search is to compare
values.

.. important:: Types Must Match

   The type on both sides of the operator must be equivalent. For
   example, comparing an ObjectId with string will result in a precondition
   failure with a message like:

   .. code-block::
      :copyable: false

      "Expected object of type object id for property 'id' on object of type
      'User', but received: 11223344556677889900aabb (Invalid value)"

   You can compare any numeric type with any other numeric type,
   including decimal, float, and Decimal128.

.. list-table::
   :header-rows: 1
   :widths: 30 70

   * - Operator
     - Description

   * - ``BETWEEN {number1, number2}``
     - Evaluates to ``true`` if the left-hand numerical or date expression
       is between or equal to the right-hand range. For dates, this evaluates
       to ``true`` if the left-hand date is within the right-hand date range.

   * - | ``==``, ``=``
     - Evaluates to ``true`` if the left-hand expression is equal
       to the right-hand expression.

   * - | ``>``
     - Evaluates to ``true`` if the left-hand numerical or date expression
       is greater than the right-hand numerical or date expression.
       For dates, this evaluates to ``true`` if the left-hand date is later
       than the right-hand date.

   * - | ``>=``
     - Evaluates to ``true`` if the left-hand numerical or date expression
       is greater than or equal to the right-hand numerical or date expression.
       For dates, this evaluates to ``true`` if the left-hand date is later than
       or the same as the right-hand date.

   * - | ``IN``
     - Evaluates to ``true`` if the left-hand expression is in the
       right-hand list. This is equivalent to and used as a shorthand
       for ``== ANY``.

   * - | ``<``
     - Evaluates to ``true`` if the left-hand numerical or date expression
       is less than the right-hand numerical or date expression.
       For dates, this evaluates to ``true`` if the left-hand date is earlier
       than the right-hand date.

   * - | ``<=``
     - Evaluates to ``true`` if the left-hand numeric expression is less than
       or equal to the right-hand numeric expression. For dates, this evaluates
       to ``true`` if the left-hand date is earlier than or the same
       as the right-hand date.

   * - | ``!=``, ``<>``
     - Evaluates to ``true`` if the left-hand expression is not equal
       to the right-hand expression.

.. example::

   The following example uses Realm Query Language's comparison operators to:

   - Find high priority tasks by comparing the value of the ``priority``
     property value with a threshold number, above which priority can be considered high.
   - Find long-running tasks by seeing if the ``progressMinutes`` property
     is at or above a certain value.
   - Find unassigned tasks by finding tasks where the ``assignee`` property
     is equal to ``null``.
   - Find tasks within a certain time range by finding tasks where the
     ``progressMinutes`` property is between two numbers.

   .. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.comparison-operators.js
      :language: javascript

.. _rql-logical-operators:

Logical Operators
-----------------

Make compound predicates using logical operators.

.. list-table::
   :header-rows: 1
   :widths: 30 70

   * - Operator
     - Description

   * - | ``AND``
       | ``&&``
     - Evaluates to ``true`` if both left-hand and right-hand expressions are ``true``.

   * - | ``NOT``
       | ``!``
     - Negates the result of the given expression.

   * - | ``OR``
       | ``||``
     - Evaluates to ``true`` if either expression returns ``true``.

.. example::

   We can use the query language's logical operators to find
   all of Ali's completed tasks. That is, we find all tasks
   where the ``assignee`` property value is equal to 'Ali' AND
   the ``isComplete`` property value is ``true``:

   .. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.logical-operators.js
      :language: javascript

.. _rql-string-operators:

String Operators
----------------

Compare string values using these string operators.
Regex-like wildcards allow more flexibility in search.

.. note::

   You can use the following modifiers with the string operators:

   - ``[c]`` for case insensitivity.

     .. code-block:: javascript

        "name CONTAINS[c] 'a'"

.. list-table::
   :header-rows: 1
   :widths: 40 60

   * - Operator
     - Description

   * - | ``BEGINSWITH``
     - Evaluates to ``true`` if the left-hand string expression begins with
       the right-hand string expression. This is similar to ``contains``,
       but only matches if the right-hand string expression is found
       at the beginning of the left-hand string expression.

   * - | ``CONTAINS``
     - Evaluates to ``true`` if the right-hand string expression
       is found anywhere in the left-hand string expression.

   * - | ``ENDSWITH``
     - Evaluates to ``true`` if the left-hand string expression ends
       with the right-hand string expression. This is similar to ``contains``,
       but only matches if the left-hand string expression is found
       at the very end of the right-hand string expression.

   * - | ``LIKE``
     - Evaluates to ``true`` if the left-hand string expression
       matches the right-hand string wildcard string
       expression. A wildcard string expression is a string
       that uses normal characters with two special wildcard
       characters:

       - The ``*`` wildcard matches zero or more of any character
       - The ``?`` wildcard matches any character.

       For example, the wildcard string "d?g" matches "dog",
       "dig", and "dug", but not "ding", "dg", or "a dog".

   * - | ``==``, ``=``
     - Evaluates to ``true`` if the left-hand string is lexicographically equal
       to the right-hand string.

   * - | ``!=``, ``<>``
     - Evaluates to ``true`` if the left-hand string is not lexicographically
       equal to the right-hand string.

.. example::

   We use the query engine's string operators to find:

   - Projects with a name starting with the letter 'e'
   - Projects with names that contain 'ie'

   .. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.string-operators.js
      :language: javascript

.. _rql-objectid-uuid-operators:

ObjectId and UUID Operators
---------------------------

Query :manual:`BSON ObjectIds </reference/method/ObjectId>` and
:manual:`UUIDs </reference/method/UUID/>`.
These data types are often used as primary keys.

To query with ObjectIds, use a parameterized query. Pass the ObjectId or UUID
you're querying against as the argument.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.oid-literal.js
   :language: js

You can also put a string representation of the ObjectId you're evaluating
in ``oid(<ObjectId String>)``.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.oid.js
   :language: js

To query with UUIDs, put a string representation of the UUID you're evaluating
in ``uuid(<UUID String>)``.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.uuid.js
   :language: js

.. list-table::
   :header-rows: 1
   :widths: 40 60

   * - Operator
     - Description

   * - | ``==``, ``=``
     - Evaluates to ``true`` if the left-hand value is equal
       to the right-hand value.

   * - | ``!=``, ``<>``
     - Evaluates to ``true`` if the left-hand value is not equal
       to the right-hand value.

.. _rql-arithmetic-operators:

Arithmetic Operators
--------------------

Perform basic arithmetic in one side of a RQL expression when evaluating
numeric data types.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.basic-arithmetic.js
   :language: js

You can also use multiple object properties together in a mathematic operation.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.arithmetic-obj-properties.js
   :language: js

.. list-table::
   :header-rows: 1
   :widths: 40 60

   * - Operator
     - Description

   * - | ``*``
     - Multiplication.
   * - | ``/``
     - Division.
   * - | ``+``
     - Addition.
   * - | ``-``
     - Subtraction.
   * - | ``()``
     - Group expressions together.

.. _rql-type-operator:

Type Operator
-------------

Check the type of a property using the ``@type`` operator.
You can only use the type operator with mixed types and dictionaries.

Evaluate the property against a string representation of the data type name.
Refer to SDK documentation on the mapping from the SDK language's data types
to Realm data types.

.. list-table::
   :header-rows: 1
   :widths: 40 60

   * - Operator
     - Description

   * - ``@type``
     - Check if type of a property is the property name as a string.
       Use ``==`` and ``!=`` to compare equality.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.type-operator.js
   :language: js

.. _rql-dictionary-operators:

Dictionary Operators
--------------------

Compare dictionary values using these dictionary operators.

.. list-table::
   :header-rows: 1
   :widths: 40 60

   * - Operator
     - Description

   * - ``@values``
     - Returns objects that have the value specified in the right-hand expression.

   * - ``@keys``
     - Returns objects that have the key specified in the right-hand expression.

   * - ``@size``, ``@count``
     - The number of elements in a dictionary.

   * - ``Dictionary['key']``
     - Access the value at a key of a dictionary.

   * - ``ALL | ANY | NONE <property>.@type``
     - Checks if the dictionary contains properties of certain type.

You can also use dictionary operators in combination with
:ref:`comparison operators <rql-comparison-operators>` to filter objects
based on dictionary keys and values. The following examples show some ways
to use dictionary operators with comparison operators. All examples query
a collection of Realm objects with a dictionary property named ``dict``.

.. example::

   The following examples use various dictionary operators.

   .. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.dictionary-operators.js
      :language: js

.. _rql-date-operators:

Date Operators
--------------

Query date types in a realm.

Generally, you should use a parameterized query to pass a date data type
from the SDK language you are using to a query.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.date-parameterized-query.js
   :language: js

You can also represent dates the following ways:

- ``YYYY-MM-DD@HH:mm:ss:nnnnnnnnnn`` (year-month-day@hours:minutes:seconds:nanoseconds), UTC.
  You can also use ``T`` instead of ``@`` to separate the date from the time.
- ``Ts:n``, where ``s`` is the number of seconds and n the number of nanoseconds
  since :wikipedia:`unix epoch <Unix_time>`.

Date supports :ref:`comparison operators <rql-comparison-operators>`.

.. example::

   The following example shows how to compare dates with their different
   representations.

   .. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.date-alt-representation.js
      :language: js

.. _rql-aggregate-operators:

Aggregate Operators
-------------------

Apply an aggregate operator to a collection property of a Realm
object. Aggregate operators traverse a collection and reduce it to a
single value.

.. list-table::
   :header-rows: 1
   :widths: 40 60

   * - Operator
     - Description

   * - | ``@avg``
     - Evaluates to the average value of a given numerical property across
       a collection. If any values are ``null``, they are not counted
       in the result.

   * - | ``@count``
     - Evaluates to the number of objects in the given collection.

   * - | ``@max``
     - Evaluates to the highest value of a given numerical property
       across a collection. ``null`` values are ignored.

   * - | ``@min``
     - Evaluates to the lowest value of a given numerical property
       across a collection. ``null`` values are ignored.

   * - | ``@sum``
     - Evaluates to the sum of a given numerical property across a collection,
       excluding ``null`` values.

.. example::

   These examples all query for projects containing tasks that meet
   this criteria:

   - Projects with average task priority above 5.
   - Projects with a task whose priority is less than 5.
   - Projects with a task whose priority is greater than 5.
   - Projects with more than 5 tasks.
   - Projects with long-running tasks.

   .. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.aggregate-operators.js
      :language: javascript

.. _rql-collection-operators:

Collection Operators
--------------------

A **collection operator** lets you query list properties within a collection of objects.
Collection operators filter a collection by applying a predicate
to every element of a given list property of the object.
If the predicate returns true, the object is included in the output collection.

.. list-table::
   :header-rows: 1
   :widths: 30 70

   * - Operator
     - Description

   * - ``ALL``
     - Returns objects where the predicate evaluates to ``true`` for all objects
       in the collection.

   * - ``ANY``, ``SOME``
     - Returns objects where the predicate evaluates to ``true`` for any objects
       in the collection.

   * - ``NONE``
     - Returns objects where the predicate evaluates to false for all objects
       in the collection.

.. example::

   This example uses collection operators to find projects that contain tasks
   matching certain criteria:

   .. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.set-operators.js
      :language: js

.. _rql-list-queries:

List Comparisons
----------------

You can use :ref:`comparison operators <rql-comparison-operators>` and
:ref:`collection operators <rql-collection-operators>` to filter based
on lists of data.

You can compare any type of valid list. This includes:

- collections of Realm objects, which let you filter against other data
  in the realm.

  .. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.list-comparisons-collection.js
     :language: js

- lists defined directly in the query, which let you filter against
  static data. You define static lists as a comma-separated list of
  literal values enclosed in opening (``{``) and closing (``}``) braces.

  .. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.list-comparisons-static.js
     :language: js

- native list objects passed in a :ref:`parameterized expression
  <rql-parameterized-queries>`, which let you pass application data
  directly to your queries.

  .. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.list-comparisons-parameterized.js
     :language: js

If you do not define a collection operator, a list expression defaults
to the ``ANY`` operator.

.. example::
   
   These two list queries are equivalent:
   
   - ``age == ANY {18, 21}``
   - ``age == {18, 21}``

   Both of these queries return objects with an age property equal to
   either 18 or 21. You could also do the opposite by returning objects
   only if the age is not equal to either 18 or 21:

   - ``age == NONE {18, 21}``

The following table includes examples that illustrate how collection
operators interact with lists and comparison operators:

.. list-table::
   :widths: 45 10 45

   * - Expression
     - Match?
     - Reason
     
   * - ``ANY {1, 2, 3} > ALL {1, 2}``
     - true
     - A value on the left (3) is greater than some value on the right (both 1 and 2)
     
   * - ``ANY {1, 2, 3} == NONE {1, 2}``
     - true
     - 3 does not match either of 1 or 2
     
   * - ``ANY {4, 8} == ANY {5, 9, 11}``
     - false
     - Neither 4 nor 8 matches any value on the right (5, 9 or 11)
     
   * - ``ANY {1, 2, 7} <= NONE {1, 2}``
     - true
     - A value on the left (7) is not less than or equal to both 1 and 2
     
   * - ``ALL {1, 2} IN ANY {1, 2, 3}``
     - true
     - Every value on the left (1 and 2) is equal to 1, 2 or 3
     
   * - ``ALL {3, 1, 4, 3} == NONE {1, 2}``
     - false
     - 1 matches a value in the NONE list (1 or 2)
     
   * - ``ALL {} in ALL {1, 2}``
     - true
     - An empty list matches all lists
     
   * - ``NONE {1, 2, 3, 12} > ALL {5, 9, 11}``
     - false
     - 12 is bigger than all values on the right (5, 9, and 11)
     
   * - ``NONE {4, 8} > ALL {5, 9, 11}``
     - true
     - 4 and 8 are both less than some value on the right (5, 9, or 11)
     
   * - ``NONE {0, 1} < NONE {1, 2}``
     - true
     - 0 and 1 are both less than none of 1 and 2

.. _rql-backlinks:

Backlink Queries
----------------

A backlink is an inverse relationship link that lets you look up objects
that reference another object. Backlinks use the to-one and to-many
relationships defined in your object schemas but reverse the direction.
Every relationship that you define in your schema implicitly has a
corresponding backlink.

You can access backlinks in queries using the
``@links.<ObjectType>.<PropertyName>`` syntax, where ``<ObjectType>``
and ``<PropertyName>`` refer to a specific property on an object type
that references the queried object type.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.backlinks-atLinks.js
     :language: js

You can also define a ``linkingObjects`` property to explicitly include
the backlink in your data model. This lets you reference the backlink
through an assigned property name using standard :ref:`dot notation
<rql-dot-notation>`.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.backlinks-linkingObjects.js
     :language: js

The result of a backlink is treated like a collection and supports
:ref:`collection operators <rql-collection-operators>`.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.backlinks-collection-operators.js
     :language: js

You can use :ref:`aggregate operators <rql-aggregate-operators>` on the backlink collection.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.backlinks-aggregate-operators.js
     :language: js

You can query the count of all relationships that point to an object by
using the ``@count`` operator directly on ``@links``.

.. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.backlinks-atCount.js
     :language: js

.. _rql-subqueries:

Subqueries
----------

Iterate through list properties with another query using the
``SUBQUERY()`` predicate function.

Subqueries are useful for the following scenarios:

- Matching each object in a list property on multiple conditions
- Counting the number of objects that match a subquery


``SUBQUERY()`` has the following structure:

.. code-block:: js

   SUBQUERY(<collection>, <variableName>, <predicate>)

- ``collection``: The name of the property to iterate through
- ``variableName``: A variable name of the element to use in the subquery
- ``predicate``: The subquery predicate.
  Use the variable specified by ``variableName`` to refer to the
  currently-iterated element.

A subquery iterates through the given collection and checks the given predicate
against each object in the collection. The predicate can refer to the current
iterated object with the variable name passed to ``SUBQUERY()``.

A subquery expression resolves to a list of objects.
Realm Database only supports the ``@count`` aggregate operator on the result
of a subquery. This allows you to count how many objects in the subquery
input collection matched the predicate.

You can use the count of the subquery result as you would any other number
in a valid expression. In particular, you can compare the count with the
number ``0`` to return all matching objects.


.. example::

   The following example shows two subquery filters on a collection of projects.

   .. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.subquery.js
      :language: js

.. _rql-sort-distinct-limit:

Sort, Distinct & Limit
----------------------

Sort and limit the results collection of your query using additional operators.

.. list-table::
   :header-rows: 1
   :widths: 40 60

   * - Operator
     - Description

   * - ``SORT``
     - Specify the name of the property to compare. You can optionally
       specify ascending (``ASC``) or descending (``DESC``) order.
       If you specify multiple SORT fields, the query sorts by the first
       field, and then the second. For example, if you ``SORT (priority, name)``,
       the query returns sorted by priority, and then by name when priority
       value is the same.

   * - ``DISTINCT``
     - Specify a name of the property to compare. Remove duplicates
       for that property in the results collection. If you specify multiple
       DISTINCT fields, the query removes duplicates by the first field, and
       then the second. For example, if you ``DISTINCT (name, assignee)``,
       the query only removes duplicates where the values of both properties
       are the same.

   * - ``LIMIT``
     - Limit the results collection to the specified number.

.. example::

   Use the query engine's sort, distinct, and limit operators to find tasks
   where the assignee is Ali:

   - Sorted by priority in descending order
   - Enforcing uniqueness by name
   - Limiting the results to 5 tasks

   .. literalinclude:: /examples/generated/realm-query-language/realm-query-language.snippet.sort-distinct-limit.js
      :language: javascript

.. _flexible-sync-rql-limitations:

Flexible Sync RQL Limitations
-----------------------------

.. include:: /includes/flex-sync-limitations.rst