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$ifNull (aggregation)

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$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.

Syntax:

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

In MongoDB 4.4 and earlier versions, $ifNull only accepts a single input expression:

{
$ifNull: [
<input-expression>,
<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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