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$ifNull (aggregation)

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  • Definition
  • Compatibility
  • Syntax
  • Examples
$ifNull

Changed in version 5.0.

The $ifNull expression evaluates input expressions for null values and returns:

  • The first non-null input expression value found.

  • A replacement expression value if all input expressions evaluate to null.

$ifNull treats undefined values and missing fields as null.

You can use $ifNull for deployments hosted in the following environments:

  • MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

{
$ifNull: [
<input-expression-1>,
...
<input-expression-n>,
<replacement-expression-if-null>
]
}

This inventory collection is used in the examples:

db.inventory.insertMany( [
{ "_id" : 1, "item" : "buggy", description: "toy car", "quantity" : 300 },
{ "_id" : 2, "item" : "bicycle", description: null, "quantity" : 200 },
{ "_id" : 3, "item" : "flag" }
] )

The following example uses $ifNull to return:

  • description if it is non-null.

  • "Unspecified" string if description is null or missing.

db.inventory.aggregate(
[
{
$project: {
item: 1,
description: { $ifNull: [ "$description", "Unspecified" ] }
}
}
]
)

Output:

{ "_id" : 1, "item" : "buggy", "description" : "toy car" }
{ "_id" : 2, "item" : "bicycle", "description" : "Unspecified" }
{ "_id" : 3, "item" : "flag", "description" : "Unspecified" }

New in version 5.0.

The following example uses $ifNull to return:

  • description if it is non-null.

  • quantity if description is null or missing and quantity is non-null.

  • "Unspecified" string if description and quantity are both null or missing.

db.inventory.aggregate(
[
{
$project: {
item: 1,
value: { $ifNull: [ "$description", "$quantity", "Unspecified" ] }
}
}
]
)

Output:

{ "_id" : 1, "item" : "buggy", "value" : "toy car" }
{ "_id" : 2, "item" : "bicycle", "value" : 200 }
{ "_id" : 3, "item" : "flag", "value" : "Unspecified" }

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