$ifNull (aggregation)¶

$ifNull¶

Takes an array with two expressions. $ifNull returns the first expression if it evaluates to a non-null value. Otherwise, $ifNull returns the second expression’s value.

Use the $ifNull operator with the following syntax:

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{ $ifNull: [ <expression>, <replacement-if-null> ] }

Both values in the array specified to $ifNull must be valid MongoDB aggregation expressions or document fields. Do not use JavaScript in any aggregation statements, including $ifNull.

Example

The following aggregation on the offSite collection groups by the location field and returns a count for each location. If the location field contains null or does not exist, the $ifNull returns "Unspecified" as the value. MongoDB assigns the returned value to _id in the aggregated document.

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db.offSite.aggregate(
   [
      {
         $group: {
            _id: { $ifNull: [ "$location", "Unspecified" ] },
            count: { $sum: 1 }
         }
      }
   ]
)

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