$ifNull (aggregation)

Definition

$ifNull

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>
   ]
}

Examples

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" }
] )

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

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

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

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