$round (aggregation)¶

Definition定义¶

$round¶

New in version 4.2..

$round rounds a number to a whole integer or to a specified decimal place.将数字舍入为整数或指定的小数点。

$round has the following syntax:语法如下所示：

{ $round : [ <number>, <place> ] }

Field字段	Type类型	Description描述
<number>	number	Can be any valid expression that resolves to a number. Specifically, the expression must resolve to an integer, double, decimal, or long. $round returns an error if the expression resolves to a non-numeric data type.
<place>	integer	Optional Can be any valid expression that resolves to an integer between -20 and 100, exclusive. e.g. -20 < place < 100. Defaults to 0 if unspecified. If <place> resolves to a positive integer, $round rounds to <place> decimal places. For example, $round : [1234.5678, 2] rounds to two decimal places and returns 1234.57. If <place> resolves to a negative integer, $round rounds using the digit <place> to the left of the decimal. For example, $round : [1234.5678, -2] uses the 2nd digit to the left of the decimal (3) and returns 1200. If the absolute value of <place> equals or exceeds the number of digits to the left of the decimal, $round returns 0. For example, $round : [ 1234.5678, -4] specifies the fourth digit to the left of the decimal. This equals the number of digits left of the decimal and returns 0. If <place> resolves to 0, $round rounds using the first digit to the right of the decimal and returns rounded integer value. For example, $round : [1234.5678, 0] returns 1234.

Behavior行为¶

Example示例¶

A collection named samples contains the following documents:

{ _id: 1, value: 19.25 }
{ _id: 2, value: 28.73 }
{ _id: 3, value: 34.32 }
{ _id: 4, value: -45.39 }

• The following aggregation returns value rounded to the first decimal place:

  db.samples.aggregate([
     { $project: { roundedValue: { $round: [ "$value", 1 ] } } }
  ])

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

  { "_id" : 1, "roundedValue" : 19.2 }
  { "_id" : 2, "roundedValue" : 28.7 }
  { "_id" : 3, "roundedValue" : 34.3 }
  { "_id" : 4, "roundedValue" : -45.4 }

• The following aggregation returns value rounded using the first digit to the left of the decimal:

  db.samples.aggregate([
     { $project: { roundedValue: { $round: [ "$value", -1 ] } } }
  ])

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

  { "_id" : 1, "roundedValue" : 10 }
  { "_id" : 2, "roundedValue" : 20 }
  { "_id" : 3, "roundedValue" : 30 }
  { "_id" : 4, "roundedValue" : -50 }

• The following aggregation returns value rounded to the whole integer:

  db.samples.aggregate([
     { $project: { roundedValue: { $round: [ "$value", 0 ] } } }
  ])

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

  { "_id" : 1, "roundedValue" : 19 }
  { "_id" : 2, "roundedValue" : 29 }
  { "_id" : 3, "roundedValue" : 34 }
  { "_id" : 4, "roundedValue" : -45 }