$exists
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Definition
$exists-
Syntax:
{ field: { $exists: <boolean> } }When
<boolean>is true,$existsmatches the documents that contain the field, including documents where the field value isnull. If<boolean>is false, the query returns only the documents that do not contain the field.MongoDB $exists does not correspond to SQL operator
exists. For SQLexists, refer to the$inoperator.
See also
Examples
Exists and Not Equal To
Consider the following example:
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:
{ 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 }:
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:
{ 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 }