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$exists

=======

.. default-domain:: mongodb

Definition

----------

.. query:: $exists

*Syntax*: ``{ field: { $exists: <boolean> } }``

When ``<boolean>`` is true, :query:`$exists` matches the documents that

contain the field, including documents where the field value is

``null``. If ``<boolean>`` is false, the query returns only the

documents that do not contain the field.

MongoDB `$exists` does **not** correspond to SQL operator

``exists``. For SQL ``exists``, refer to the :query:`$in`

operator.

.. seealso:: :query:`$nin`, :query:`$in`, and

:ref:`faq-developers-query-for-nulls`.

Examples

--------

Exists and Not Equal To

~~~~~~~~~~~~~~~~~~~~~~~

Consider the following example:

.. code-block:: javascript

db.inventory.find( { qty: { $exists: true, $nin: [ 5, 15 ] } } )

This query will select all documents in the ``inventory`` collection

where the ``qty`` field exists *and* its value does not equal ``5`` or

``15``.

Null Values

~~~~~~~~~~~

Given a collection named ``records`` with the following documents:

.. code-block:: javascript

{ a: 5, b: 5, c: null }

{ a: 3, b: null, c: 8 }

{ a: null, b: 3, c: 9 }

{ a: 1, b: 2, c: 3 }

{ a: 2, c: 5 }

{ a: 3, b: 2 }

{ a: 4 }

{ b: 2, c: 4 }

{ b: 2 }

{ c: 6 }

Consider the output of the following queries:

**Query**:

.. code-block:: javascript

db.records.find( { a: { $exists: true } } )

**Result**:

.. code-block:: javascript

{ a: 5, b: 5, c: null }

{ a: 3, b: null, c: 8 }

{ a: null, b: 3, c: 9 }

{ a: 1, b: 2, c: 3 }

{ a: 2, c: 5 }

{ a: 3, b: 2 }

{ a: 4 }

**Query**:

.. code-block:: javascript

db.records.find( { b: { $exists: false } } )

**Result**:

.. code-block:: javascript

{ a: 2, c: 5 }

{ a: 4 }

{ c: 6 }

**Query**:

.. code-block:: javascript

db.records.find( { c: { $exists: false } } )

**Result**:

.. code-block:: javascript

{ a: 3, b: 2 }

{ a: 4 }

{ b: 2 }