Docs Home → MongoDB Manual

$trunc (aggregation)

On this page本页内容

Definition定义

$trunc

Changed in version 4.2..4.2.版更改。

$trunc truncates a number to a whole integer or to a specified decimal place.将数字截断为整整数或指定的小数位数。

MongoDB 4.2 adds the following syntax for $trunc:MongoDB 4.2为$trunc添加了以下语法：

{ $trunc : [ <number>, <place> ] }
Field字段Type类型Description描述
<number>numberCan be any valid expression that resolves to a number. 可以是解析为数字的任何有效表达式。Specifically, the expression must resolve to an integer, double, decimal, or long.具体来说，表达式必须解析为integer、double、decimal或long。
$trunc returns an error if the expression resolves to a non-numeric data type. 如果表达式解析为非数字数据类型，则返回错误。
<place>integerOptional. 可选的。Can be any valid expression that resolves to an integer between -20 and 100, exclusive. e.g. -20 < place < 100. 可以是解析为介于-20和100之间的整数的任何有效表达式。例如-20 < place < 100。Defaults to 0 if unspecified. 如果未指定，则默认为0。
  • If <place> resolves to a positive integer, $trunc truncates to <place> decimal places.如果<place>解析为正整数，$trunc将截断为<place>小数位。
    For example, $trunc : [1234.5678, 2] truncates to two decimal places and returns 1234.56.例如，$trunc : [1234.5678, 2]截断到小数点后两位并返回1234.56。
  • If <place> resolves to a negative integer, $trunc replaces <place> digits left of the decimal with 0.如果<place>解析为负整数，$trunc将小数点左侧的<place>数字替换为0。
    For example, $trunc : [1234.5678, -2] replaces to two digits left of the decimal with 0 and returns 1200.例如，$trunc : [1234.5678, -2]将小数点后两位替换为0，并返回1200。
    If the absolute value of <place> exceeds the number of digits to the left of the decimal, $trunc returns 0.如果<place>的绝对值超过小数点左侧的位数，$trunc将返回0。
    For example, $trunc : [ 1234.5678, -5] specifies the fifth digit left of the decimal. 例如，$trunc : [ 1234.5678, -5]指定小数点左边的第五位。This exceeds the number of digits left of the decimal and returns 0.这超过了小数点的剩余位数，并返回0。
  • If <place> resolves to 0, $trunc truncates all digits to the right of the decimal and returns the whole integer value.如果<place>解析为0，$trunc将截断小数点右侧的所有数字，并返回整个整数值。
    For example, $trunc : [1234.5678, 0] returns 1234例如，$trunc : [1234.5678, 0]返回1234。

Prior to MongoDB 4.2, $trunc truncated the input value to the whole integer. MongoDB 4.2 continues supporting the pre-4.2 syntax and behavior:在MongoDB 4.2之前，$trunc将输入值截断为整数。MongoDB 4.2继续支持4.2之前的语法和行为：

{ $trunc: <number> }

The <number> expression can be any valid expression as long as it resolves to a number. <number>表达式可以是任何有效的表达式，只要它解析为数字即可。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

$trunc does not round the truncated data. 不舍入截断的数据。To round input values to a specified place, use the $round expression.若要将输入值四舍五入到指定位置，请使用$round表达式。

Returned Data Type返回的数据类型

If truncating to a specific decimal place, the data type returned by $trunc matches the data type of the input expression or value.如果截断到特定的小数点，$trunc返回的数据类型将与输入表达式或值的数据类型匹配。

If truncating to a whole integer value, $trunc returns an integer.如果截断为整数值，$trunc将返回一个整数。

null, NaN, and +/- Infinity

  • If the argument resolves to a value of null or refers to a field that is missing, $trunc returns null.如果参数解析为null值或引用了缺失的字段，$trunc将返回null。
  • If the argument resolves to NaN, $trunc returns NaN.如果参数解析为NaN，$trunc将返回NaN。
  • If the argument resolves to negative or positive infinity, $trunc returns negative or positive infinity respectively.如果参数解析为负无穷大或正无穷大，$trunc将分别返回负无穷大和正无穷大。
Example示例Results结果
{ $trunc: [ NaN, 1] }NaN
{ $trunc: [ null, 1] }null
{ $trunc : [ Infinity, 1 ] }Infinity
{ $trunc : [ -Infinity, 1 ] }-Infinity

Example实例

Create a collection named samples with the following documents:使用以下文档创建一个名为samples的集合：

db.samples.insertMany(
   [
      { _id: 1, value: 19.25 },
      { _id: 2, value: 28.73 },
      { _id: 3, value: 34.32 },
      { _id: 4, value: -45.34 }
   ]
)
  • The following aggregation returns value truncated to the first decimal place:以下聚合返回截断到小数点后第一位的value：

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

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

    { "_id" : 1, "truncatedValue" : 19.2 }
    { "_id" : 2, "truncatedValue" : 28.7 }
    { "_id" : 3, "truncatedValue" : 34.3 }
    { "_id" : 4, "truncatedValue" : -45.3 }
  • The following aggregation returns value truncated to the first place:以下聚合返回截断到第一位的value：

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

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

    { "_id" : 1, "truncatedValue" : 10 }
    { "_id" : 2, "truncatedValue" : 20 }
    { "_id" : 3, "truncatedValue" : 30 }
    { "_id" : 4, "truncatedValue" : -40 }
  • The following aggregation returns value truncated to the whole integer:以下聚合返回截断为整整数的value：

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

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

    { "_id" : 1, "truncatedValue" : 19 }
    { "_id" : 2, "truncatedValue" : 28 }
    { "_id" : 3, "truncatedValue" : 34 }
    { "_id" : 4, "truncatedValue" : -45 }