$max (aggregation)¶

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Definition
Behavior
Examples

Definition¶

$max¶

Returns the maximum value. $max compares both value and type, using the specified BSON comparison order for values of different types.

在 3.2 版更改: $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, $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:

{ "_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 }