$exists¶

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Definition
Examples

Definition¶

$exists¶

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

When <boolean> is true, $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. [1]

MongoDB $exists does not correspond to SQL operator exists. For SQL exists, refer to the $in operator.

Examples¶

Exists and Not Equal To¶

Consider the following example:

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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¶

The following examples uses a collection named records with the following documents:

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{ 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 }

`$exists: true`¶

The following query specifies the query predicate a: { $exists: true }:

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

The results consist of those documents that contain the field a, including the document whose field a contains a null value:

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{ 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 }

`$exists: false`¶

The following query specifies the query predicate b: { $exists: false }:

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

The results consist of those documents that do not contain the field b:

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{ a: 2, c: 5 }
{ a: 4 }
{ c: 6 }

Starting in MongoDB 4.2, users can no longer use the query filter $type: 0 as a synonym for $exists:false. To query for null or missing fields, see Query for Null or Missing Fields.

Use a Sparse Index to Improve `$exists` Performance¶

The following scenario is not optimal because all of the collection’s documents are examined:

You use a query to retrieve or count documents, and
use field: { $exists: true }, and
the field has a non-sparse index or does not have an index.

To improve performance, create a sparse index on the field as shown in the following scenario:

Create a stockSales collection:

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db.stockSales.insertMany( [
   { _id: 0, symbol: "ABC", auditDate: new Date( "2021-05-18T16:12:23Z" ) },
   { _id: 1, symbol: "ABC", auditDate: new Date( "2021-04-21T11:34:45Z" ) },
   { _id: 2, symbol: "DEF", auditDate: new Date( "2021-02-24T15:11:32Z" ) },
   { _id: 3, symbol: "DEF", auditDate: null },
   { _id: 4, symbol: "DEF", auditDate: new Date( "2021-07-13T18:32:54Z" ) },
   { _id: 5, symbol: "XYZ" }
] )

The document with an _id of:

3 has a null auditDate value.
5 is missing the auditDate value.

Create a sparse index on the auditDate field:

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db.getCollection( "stockSales" ).createIndex(
   { auditDate: 1 },
   { name: "auditDateSparseIndex", sparse: true }
)

The following example counts the documents where the auditDate field has a value (including null) and uses the sparse index:
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```
db.stockSales.countDocuments( { auditDate: { $exists: true } } )
```
The example returns 5. The document that is missing the auditDate value is not counted.

Tip

If you only need documents where the field has a non-null value, you:

Can use $ne: null instead of $exists: true.
Do not need a sparse index on the field.

For example, using the stockSales collection:

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db.stockSales.countDocuments( { auditDate: { $ne: null } } )

The example returns 4. Documents that are missing the auditDate value or have a null auditDate value are not counted.

$exists¶

Definition¶

Examples¶

Exists and Not Equal To¶

Null Values¶

$exists: true¶

$exists: false¶

Use a Sparse Index to Improve $exists Performance¶

`$exists: true`¶

`$exists: false`¶

Use a Sparse Index to Improve `$exists` Performance¶