distinct

Array Fields

If the value of the specified field is an array, distinct considers each element of the array as a separate value.

For instance, if a field has as its value [ 1, [1], 1 ], then distinct considers 1, [1], and 1 as separate values.

Starting in MongoDB 6.0, the distinct command returns the same results for collections and views when using arrays.

For examples, see:

• Return Distinct Values for an Array Field

• Arrays in Collections and Views

Index Use

When possible, distinct operations can use indexes.

Indexes can also cover distinct operations. See Covered Query for more information on queries covered by indexes.

Transactions

To perform a distinct operation within a transaction:

• For unsharded collections, you can use the db.collection.distinct() method/the distinct command as well as the aggregation pipeline with the $group stage.

• For sharded collections, you cannot use the db.collection.distinct() method or the distinct command.

  To find the distinct values for a sharded collection, use the aggregation pipeline with the $group stage instead. See Distinct Operation for details.

Client Disconnection

Starting in MongoDB 4.2, if the client that issued distinct disconnects before the operation completes, MongoDB marks distinct for termination using killOp.

Replica Set Member State Restriction

Starting in MongoDB 4.4, to run on a replica set member, distinct operations require the member to be in PRIMARY or SECONDARY state. If the member is in another state, such as STARTUP2, the operation errors.

In previous versions, the operations also run when the member is in STARTUP2. The operations wait until the member transitioned to RECOVERING.

Index Filters and Collations

Starting in MongoDB 6.0, an index filter uses the collation previously set using the planCacheSetFilter command.

Return Distinct Values for a Field

The following example returns the distinct values for the field dept from all documents in the inventory collection:

db.runCommand ( { distinct: "inventory", key: "dept" } )

The command returns a document with a field named values that contains the distinct dept values:

{
   "values" : [ "A", "B" ],
   "ok" : 1
}

Return Distinct Values for an Embedded Field

The following example returns the distinct values for the field sku, embedded in the item field, from all documents in the inventory collection:

db.runCommand ( { distinct: "inventory", key: "item.sku" } )

The command returns a document with a field named values that contains the distinct sku values:

{
  "values" : [ "111", "222", "333" ],
  "ok" : 1
}

Return Distinct Values for an Array Field

The following example returns the distinct values for the field sizes from all documents in the inventory collection:

db.runCommand ( { distinct: "inventory", key: "sizes" } )

The command returns a document with a field named values that contains the distinct sizes values:

{
  "values" : [ "M", "S", "L" ],
  "ok" : 1
}

For information on distinct and array fields, see the Behavior section.

Arrays in Collections and Views

Starting in MongoDB 6.0, the distinct command returns the same results for collections and views when using arrays.

The following example creates a collection named sensor with an array of temperature values for each document:

db.sensor.insertMany( [
   { _id: 0, temperatures: [ { value: 1 }, { value: 4 } ] },
   { _id: 1, temperatures: [ { value: 2 }, { value: 8 } ] },
   { _id: 2, temperatures: [ { value: 3 }, { value: 12 } ] },
   { _id: 3, temperatures: [ { value: 1 }, { value: 4 } ] }
] )

The following example creates a view named sensorView from the sensor collection:

db.createView( "sensorView", "sensor", [] )

The following example uses distinct to return the unique values from the temperatures array in the sensor collection:

db.sensor.distinct( "temperatures.1.value" )

The 1 in temperatures.1.value specifies the temperatures array index.

Example output:

[ 4, 8, 12 ]

Example for sensorView:

db.sensorView.distinct( "temperatures.1.value" )

Example output:

• [ 4, 8, 12 ] starting in MongoDB 6.0 (identical to result returned from the sensor collection).

• [] in MongoDB versions earlier than 6.0.

Specify Query with distinct

The following example returns the distinct values for the field sku, embedded in the item field, from the documents whose dept is equal to "A":

db.runCommand ( { distinct: "inventory", key: "item.sku", query: { dept: "A"} } )

The command returns a document with a field named values that contains the distinct sku values:

{
  "values" : [ "111", "333" ],
  "ok" : 1
}

Specify a Collation

Collation allows users to specify language-specific rules for string comparison, such as rules for lettercase and accent marks.

A collection myColl has the following documents:

{ _id: 1, category: "café", status: "A" }
{ _id: 2, category: "cafe", status: "a" }
{ _id: 3, category: "cafE", status: "a" }

The following aggregation operation includes the Collation option:

db.runCommand(
   {
      distinct: "myColl",
      key: "category",
      collation: { locale: "fr", strength: 1 }
   }
)

For descriptions on the collation fields, see Collation Document.

Override Default Read Concern

To override the default read concern level of "local", use the readConcern option.

The following operation on a replica set specifies a Read Concern of "majority" to read the most recent copy of the data confirmed as having been written to a majority of the nodes.

db.runCommand(
   {
     distinct: "restaurants",
     key: "rating",
     query: { cuisine: "italian" },
     readConcern: { level: "majority" }
   }
)

To ensure that a single thread can read its own writes, use "majority" read concern and "majority" write concern against the primary of the replica set.