$trunc (aggregation)¶

Definition定义¶

$trunc¶

Changed in version 4.2..

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

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>	number	Can 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，表达式必须解析为整数、双精度、十进制或长整型。 $trunc returns an error if the expression resolves to a non-numeric data type.如果表达式解析为非数字数据类型，则$trunc返回错误。
<place>	integer	Optional Can be any valid expression that resolves to an integer between -20 and 100, exclusive. e.g. -20 < place < 100. 可选表达式可以是解析为-20到100之间（排除100，亦即-20 < place < 100）的整数的任何有效表达式。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将小数点左边的<place>位数替换为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]将小数点左边的两位数替换为0，并返回1200。 If the absolute value of <place> exceeds the number of digits to the left of the decimal, $trunc returns 0.如果<place>的绝对值超过小数点左边的位数，$trunc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数字。This exceeds the number of digits left of the decimal and returns 0.这超过了小数点后的位数，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, $trunc truncated the input value to the whole integer. 在MongoDB 4.2之前，$trunc将输入值截断为整数。MongoDB 4.2 continues supporting the pre-4.2 syntax and behavior: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，只要它解析为一个数字。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 $round expression.要将输入值舍入到指定的位置，请使用$round表达式。

Example示例¶

A collection named samples contains the following documents:名为samples的集合包含以下文档：

{ _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第一个小数位的value：

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

  The operation returns the following results: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值：

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

  The operation returns the following results: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:以下聚合返回被截断为整整数的value：

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

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

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