$max (aggregation)
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Definition
Changed in version 5.0.
Returns the maximum value. $max
compares both value and
type, using the specified BSON comparison order for values of different types.
$max
is available in these stages:
$setWindowFields
(Available starting in MongoDB 5.0)
Syntax
When used in the $bucket
, $bucketAuto
,
$group
, and $setWindowFields
stages,
$max
has this syntax:
{ $max: <expression> }
When used in other supported stages, $max
has one of two
syntaxes:
$max
has one specified expression as its operand:{ $max: <expression> } $max
has a list of specified expressions as its operand:{ $max: [ <expression1>, <expression2> ... ] }
For more information on expressions, see Expression Operators.
Behavior
Null or Missing Values
If some, but not all, documents for the $max
operation have either a null
value for the field or are
missing the field, the $max
operator only considers the
non-null and the non-missing values for the field.
If all documents for the $max
operation have null
value for the field or are missing the field, the $max
operator returns null
for the maximum value.
Array Operand
In the $group
and $setWindowFields
stages, if
the expression resolves to an array, $max
does not traverse the
array and compares the array as a whole.
In the other supported stages:
With a single expression as its operand, if the expression resolves to an array,
$max
traverses into the array to operate on the numerical elements of the array to return a single value.With a list of expressions as its operand, if any of the expressions resolves to an array,
$max
does not traverse into the array but instead treats the array as a non-numerical value.
Examples
Use in $group
Stage
Consider a sales
collection with the following documents:
{ "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-01-01T08:00:00Z") } { "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-02-03T09:00:00Z") } { "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-03T09:05:00Z") } { "_id" : 4, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-02-15T08:00:00Z") } { "_id" : 5, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T09:05:00Z") }
Grouping the documents by the item
field, the following operation
uses the $max
accumulator to compute the maximum total amount and
maximum quantity for each group of documents.
db.sales.aggregate( [ { $group: { _id: "$item", maxTotalAmount: { $max: { $multiply: [ "$price", "$quantity" ] } }, maxQuantity: { $max: "$quantity" } } } ] )
The operation returns the following results:
{ "_id" : "xyz", "maxTotalAmount" : 50, "maxQuantity" : 10 } { "_id" : "jkl", "maxTotalAmount" : 20, "maxQuantity" : 1 } { "_id" : "abc", "maxTotalAmount" : 100, "maxQuantity" : 10 }
Use in $project
Stage
A collection students
contains the following documents:
{ "_id": 1, "quizzes": [ 10, 6, 7 ], "labs": [ 5, 8 ], "final": 80, "midterm": 75 } { "_id": 2, "quizzes": [ 9, 10 ], "labs": [ 8, 8 ], "final": 95, "midterm": 80 } { "_id": 3, "quizzes": [ 4, 5, 5 ], "labs": [ 6, 5 ], "final": 78, "midterm": 70 }
The following example uses the $max
in the
$project
stage to calculate the maximum quiz scores, the
maximum lab scores, and the maximum of the final and the midterm:
db.students.aggregate([ { $project: { quizMax: { $max: "$quizzes"}, labMax: { $max: "$labs" }, examMax: { $max: [ "$final", "$midterm" ] } } } ])
The operation results in the following documents:
{ "_id" : 1, "quizMax" : 10, "labMax" : 8, "examMax" : 80 } { "_id" : 2, "quizMax" : 10, "labMax" : 8, "examMax" : 95 } { "_id" : 3, "quizMax" : 5, "labMax" : 6, "examMax" : 78 }
Use in $setWindowFields
Stage
New in version 5.0.
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 $max
in the $setWindowFields
stage to output the maximum quantity
of cake sales for each
state
:
db.cakeSales.aggregate( [ { $setWindowFields: { partitionBy: "$state", sortBy: { orderDate: 1 }, output: { maximumQuantityForState: { $max: "$quantity", window: { documents: [ "unbounded", "current" ] } } } } } ] )
In the example:
partitionBy: "$state"
partitions the documents in the collection bystate
. There are partitions forCA
andWA
.sortBy: { orderDate: 1 }
sorts the documents in each partition byorderDate
in ascending order (1
), so the earliestorderDate
is first.
output
sets themaximumQuantityForState
field to the maximumquantity
value using$max
that is run in a documents window.The window contains documents between an
unbounded
lower limit and thecurrent
document in the output. This means$max
returns the maximumquantity
for the documents between the beginning of the partition and the current document.
In this output, the maximum quantity
for CA
and WA
is shown
in the maximumQuantityForState
field:
{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"), "state" : "CA", "price" : 41, "quantity" : 162, "maximumQuantityForState" : 162 } { "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"), "state" : "CA", "price" : 13, "quantity" : 120, "maximumQuantityForState" : 162 } { "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"), "state" : "CA", "price" : 12, "quantity" : 145, "maximumQuantityForState" : 162 } { "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"), "state" : "WA", "price" : 43, "quantity" : 134, "maximumQuantityForState" : 134 } { "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"), "state" : "WA", "price" : 13, "quantity" : 104, "maximumQuantityForState" : 134 } { "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"), "state" : "WA", "price" : 14, "quantity" : 140, "maximumQuantityForState" : 140 }