$min (aggregation)

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Definition定义

$min

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

$min is available in the following stages:

In MongoDB 3.2 and earlier, $min is available in the $group stage only.

When used in the $group stage, $min has the following syntax and returns the minimum value that results from applying an expression to each document in a group of documents that share the same group by key:

{ $min: <expression> }

When used in the other supported stages, $min returns the minimum of the specified expression or list of expressions for each document and has one of two syntaxes:

  • $min has one specified expression as its operand:

    { $min: <expression> }
  • $min has a list of specified expressions as its operand:

    { $min: [ <expression1>, <expression2> ... ]  }

For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。

Behavior行为

Null or Missing Values

If some, but not all, documents for the $min operation have either a null value for the field or are missing the field, the $min operator only considers the non-null and the non-missing values for the field.

If all documents for the $min operation have null value for the field or are missing the field, the $min operator returns null for the minimum 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 other supported stages:

  • 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示例

Use in $group Stage

Consider a sales collection with the following documents:考虑一个sales集合带有以下文档：

{ "_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 $min accumulator to compute the minimum amount and minimum quantity for each grouping.

db.sales.aggregate(
   [
     {
       $group:
         {
           _id: "$item",
           minQuantity: { $min: "$quantity" }
         }
     }
   ]
)

The operation returns the following results:操作返回以下结果：

{ "_id" : "xyz", "minQuantity" : 5 }
{ "_id" : "jkl", "minQuantity" : 1 }
{ "_id" : "abc", "minQuantity" : 2 }

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 $min in the $project stage to calculate the minimum quiz scores, the minimum lab scores, and the minimum of the final and the midterm:

db.students.aggregate([
   { $project: { quizMin: { $min: "$quizzes"}, labMin: { $min: "$labs" }, examMin: { $min: [ "$final", "$midterm" ] } } }
])

The operation results in the following documents:

{ "_id" : 1, "quizMin" : 6, "labMin" : 5, "examMin" : 75 }
{ "_id" : 2, "quizMin" : 9, "labMin" : 8, "examMin" : 80 }
{ "_id" : 3, "quizMin" : 4, "labMin" : 5, "examMin" : 70 }

In the other supported stages:

  • 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.