$max (aggregation)
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Definition
-
$max
-
Returns the maximum value.
$max
compares both value and type, using the specified BSON comparison order for values of different types.Changed in version 3.2:
$max
is available in the$group
and$project
stages. In previous versions of MongoDB,$max
is available in the$group
stage only.When used in the
$group
stage,$max
has the following syntax and returns the maximum value that results from applying an expression to each document in a group of documents that share the same group by key:When used in the
$project
stage,$max
returns the maximum of the specified expression or list of expressions for each document and has one of two syntaxes:$max
has one specified expression as its operand:$max
has a list of specified expressions as its operand:
For more information on expressions, see Expressions.
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
stage, if the expression resolves to an array, $max
does not traverse the array and compares the array as a whole.
In the $project
stage:
- 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:
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.
The operation returns the following results:
Use in $project
Stage
A collection students
contains the following documents:
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:
The operation results in the following documents:
In the $project
stage:
- 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.