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- $max (aggregation)
$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:
{ $max: <expression> }
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: <expression> }
$max has a list of specified expressions as its operand:
{ $max: [ <expression1>, <expression2> ... ] }
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, $min 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, $min 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, $min 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 }
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.
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