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$denseRank (aggregation operator)

Definition

New in version 5.0.

$denseRank

Returns the document position (known as the rank) relative to other documents in the $setWindowFields stage partition.

The $setWindowFields stage sortBy field value determines the document rank. For more information on how MongoDB compares fields with different types, see BSON comparison order.

If multiple documents occupy the same rank, $denseRank places the document with the subsequent value at the next rank without any gaps (see Behavior).

$denseRank is only available in the $setWindowFields stage.

$denseRank syntax:

{ $denseRank: { } }

$denseRank does not accept any parameters.

Tip

$rank

Behavior

$rank and $denseRank differ in how they rank duplicate sortBy field values. For example, with sortBy field values of 7, 9, 9, and 10:
- $denseRank ranks the values as 1, 2, 2, and 3. The duplicate 9 values have a rank of 2, and 10 has a rank of 3. There is no gap in the ranks.
- $rank ranks the values as 1, 2, 2, and 4. The duplicate 9 values have a rank of 2, and 10 has a rank of 4. There is a gap in the ranks for 3.
Documents with a null value for a sortBy field or documents missing the sortBy field are assigned a rank based on the BSON comparison order. See the example in Rank Partitions Containing Duplicate Values, Nulls, or Missing Data.
Starting in MongoDB 8.0, null and missing field values in $denseRank and $rank sortBy operations are treated the same when calculating rankings. This change makes the behavior of denseRank and rank consistent with $sort.

See the example in Dense Rank for Duplicate, Null, and Missing Values.

Examples

Dense Rank Partitions by an Integer Field

Create a cakeSales collection that contains cake sales in the states of California (CA) and Washington (WA):

db.cakeSales.insertMany( [
   { _id: 0, type: "chocolate", orderDate: new Date("2020-05-18T14:10:30Z"),
     state: "CA", price: 13, quantity: 120 },
   { _id: 1, type: "chocolate", orderDate: new Date("2021-03-20T11:30:05Z"),
     state: "WA", price: 14, quantity: 140 },
   { _id: 2, type: "vanilla", orderDate: new Date("2021-01-11T06:31:15Z"),
     state: "CA", price: 12, quantity: 145 },
   { _id: 3, type: "vanilla", orderDate: new Date("2020-02-08T13:13:23Z"),
     state: "WA", price: 13, quantity: 104 },
   { _id: 4, type: "strawberry", orderDate: new Date("2019-05-18T16:09:01Z"),
     state: "CA", price: 41, quantity: 162 },
   { _id: 5, type: "strawberry", orderDate: new Date("2019-01-08T06:12:03Z"),
     state: "WA", price: 43, quantity: 134 }
] )

This example uses $denseRank in the $setWindowFields stage to output the quantity dense rank of the cake sales for each state:

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { quantity: -1 },
         output: {
            denseRankQuantityForState: {
               $denseRank: {}
            }
         }
      }
   }
] )

In the example:

partitionBy: "$state" partitions the documents in the collection by state. There are partitions for CA and WA.
sortBy: { quantity: -1 } sorts the documents in each partition by quantity in descending order (-1), so the highest quantity is first.

output sets the denseRankOrderDateForState field to the orderDate dense rank using $denseRank, as shown in the following results.

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "denseRankQuantityForState" : 1 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "denseRankQuantityForState" : 2 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "denseRankQuantityForState" : 3 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "denseRankQuantityForState" : 1 }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "denseRankQuantityForState" : 2 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "denseRankQuantityForState" : 3 }

Dense Rank Partitions by a Date Field

This example shows how to use dates with $denseRank in the $setWindowFields stage to output the orderDate dense rank of the cake sales for each state:

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { orderDate: 1 },
         output: {
            denseRankOrderDateForState: {
               $denseRank: {}
            }
         }
      }
   }
] )

In the example:

partitionBy: "$state" partitions the documents in the collection by state. There are partitions for CA and WA.
sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.

output sets the denseRankOrderDateForState field to the orderDate rank using $denseRank, as shown in the following results.

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "denseRankOrderDateForState" : 1 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "denseRankOrderDateForState" : 2 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "denseRankOrderDateForState" : 3 }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "denseRankOrderDateForState" : 1 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "denseRankOrderDateForState" : 2 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "denseRankOrderDateForState" : 3 }

Dense Rank for Duplicate, Null, and Missing Values

Create a cakeSalesWithDuplicates collection where:

Cake sales are placed in the state of California (CA) and Washington (WA).
Documents 6 to 8 have the same quantity and state as document 5.
Document 9 has the same quantity and state as document 4.
Document 10 has a null quantity.
Document 11 is missing the quantity.

db.cakeSalesWithDuplicates.insertMany( [
   { _id: 0, type: "chocolate", orderDate: new Date("2020-05-18T14:10:30Z"),
     state: "CA", price: 13, quantity: 120 },
   { _id: 1, type: "chocolate", orderDate: new Date("2021-03-20T11:30:05Z"),
     state: "WA", price: 14, quantity: 140 },
   { _id: 2, type: "vanilla", orderDate: new Date("2021-01-11T06:31:15Z"),
     state: "CA", price: 12, quantity: 145 },
   { _id: 3, type: "vanilla", orderDate: new Date("2020-02-08T13:13:23Z"),
     state: "WA", price: 13, quantity: 104 },
   { _id: 4, type: "strawberry", orderDate: new Date("2019-05-18T16:09:01Z"),
     state: "CA", price: 41, quantity: 162 },
   { _id: 5, type: "strawberry", orderDate: new Date("2019-01-08T06:12:03Z"),
     state: "WA", price: 43, quantity: 134 },
   { _id: 6, type: "strawberry", orderDate: new Date("2020-01-08T06:12:03Z"),
     state: "WA", price: 41, quantity: 134 },
   { _id: 7, type: "strawberry", orderDate: new Date("2020-01-01T06:12:03Z"),
     state: "WA", price: 34, quantity: 134 },
   { _id: 8, type: "strawberry", orderDate: new Date("2020-01-02T06:12:03Z"),
     state: "WA", price: 40, quantity: 134 },
   { _id: 9, type: "strawberry", orderDate: new Date("2020-05-11T16:09:01Z"),
     state: "CA", price: 39, quantity: 162 },
   { _id: 10, type: "strawberry", orderDate: new Date("2020-05-11T16:09:01Z"),
     state: "CA", price: 39, quantity: null },
   { _id: 11, type: "strawberry", orderDate: new Date("2020-05-11T16:09:01Z"),
     state: "CA", price: 39 }
] )

This example uses $denseRank in the $setWindowFields stage to output the quantity dense rank from the cakeSalesWithDuplicates collection for each state:

db.cakeSalesWithDuplicates.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { quantity: -1 },
         output: {
            denseRankQuantityForState: {
               $denseRank: {}
            }
         }
      }
   }
] )

In the example:

partitionBy: "$state" partitions the documents in the collection by state. There are partitions for CA and WA.
sortBy: { quantity: -1 } sorts the documents in each partition by quantity in descending order (-1), so the highest quantity is first.

output sets the denseRankQuantityForState field to the quantity dense rank using $denseRank.

In the following example output:

The documents with the same quantity and state have the same rank and there is no gap between the ranks. This differs from $rank that has a gap between the ranks (for an example, see Rank Partitions Containing Duplicate Values, Nulls, or Missing Data).
The document with the null quantity and then the document with the missing quantity are ranked the lowest in the output for the CA partition. This sorting is the result of the BSON comparison order, which sorts null and missing values after number values in this example.

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "denseRankQuantityForState" : 1 }
{ "_id" : 9, "type" : "strawberry", "orderDate" : ISODate("2020-05-11T16:09:01Z"),
  "state" : "CA", "price" : 39, "quantity" : 162, "denseRankQuantityForState" : 1 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "denseRankQuantityForState" : 2 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "denseRankQuantityForState" : 3 }
{ "_id" : 10, "type" : "strawberry", "orderDate" : ISODate("2020-05-11T16:09:01Z"),
  "state" : "CA", "price" : 39, "quantity" : null, "denseRankQuantityForState" : 4 }
{ "_id" : 11, "type" : "strawberry", "orderDate" : ISODate("2020-05-11T16:09:01Z"),
  "state" : "CA", "price" : 39, "denseRankQuantityForState" : 5 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "denseRankQuantityForState" : 1 }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "denseRankQuantityForState" : 2 }
{ "_id" : 6, "type" : "strawberry", "orderDate" : ISODate("2020-01-08T06:12:03Z"),
  "state" : "WA", "price" : 41, "quantity" : 134, "denseRankQuantityForState" : 2 }
{ "_id" : 7, "type" : "strawberry", "orderDate" : ISODate("2020-01-01T06:12:03Z"),
  "state" : "WA", "price" : 34, "quantity" : 134, "denseRankQuantityForState" : 2 }
{ "_id" : 8, "type" : "strawberry", "orderDate" : ISODate("2020-01-02T06:12:03Z"),
  "state" : "WA", "price" : 40, "quantity" : 134, "denseRankQuantityForState" : 2 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "denseRankQuantityForState" : 3 }

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