> For the complete documentation index, see [llms.txt](https://docs.mongoing.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.mongoing.com/aggregation/aggregation-reference/variables-in-aggregation-expressions.md).

# 聚合表达式中的变量

在本页面

* [用户变量](#user-variables)
* [系统变量](#system-variables)

[聚合表达式](https://github.com/mongodb-china/MongoDB-CN-Manual/tree/8490376c81d56eff95abbaddc6ee414b1e1c9705/docs/Aggregation/Aggregation-Reference/meta-aggregation-quick-reference.html#aggregation-expressions)可以同时使用 user-defined 和系统变量。

变量可以容纳任何[BSON 类型数据](https://github.com/mongodb-china/MongoDB-CN-Manual/tree/8490376c81d56eff95abbaddc6ee414b1e1c9705/docs/Aggregation/Aggregation-Reference/reference-bson-types.html)。要访问变量的 value，请使用带有前缀为 double 美元符号(`$$`)的变量 name 的 string。

如果变量 references 一个 object，要访问 object 中的特定字段，请使用点表示法; 即： `"$$<variable>.<field>"`。

## 用户变量

用户变量名称可以包含 ascii 字符`[_a-zA-Z0-9]`和任何 non-ascii 字符。

用户变量名必须以小写的 ascii 字母`[a-z]`或 non-ascii 字符开头。

## 系统变量

MongoDB 提供以下系统变量：

| 变量        | 描述                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
| --------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `ROOT`    | References 根文档，即： top-level 文档，当前正在聚合管道阶段中处理。                                                                                                                                                                                                                                                                                                                                                                                                                       |
| `CURRENT` | Reference 聚合管道阶段中正在处理的字段路径的开始。除非另有说明，否则所有阶段都以[CURRENT](/aggregation/aggregation-reference/variables-in-aggregation-expressions.md)开头，与[ROOT](/aggregation/aggregation-reference/variables-in-aggregation-expressions.md)相同。 [CURRENT](/aggregation/aggregation-reference/variables-in-aggregation-expressions.md)可以修改。但是，由于`$<field>`等同于`$$CURRENT.<field>`，因此重新绑定[CURRENT](/aggregation/aggregation-reference/variables-in-aggregation-expressions.md)会改变`$`访问的含义。 |
| `REMOVE`  | 一个变量，用于计算缺少的 value。允许条件排除字段。在`$projection`中，从输出中排除设置为变量[REMOVE](/aggregation/aggregation-reference/variables-in-aggregation-expressions.md)的字段。 有关其用法的示例，请参阅[有条件地排除字段](/aggregation/aggregation-reference/variables-in-aggregation-expressions.md)。 version 3.6 中的新内容。                                                                                                                                                                                              |
| `DESCEND` | [$redact](/aggregation/aggregation-reference/variables-in-aggregation-expressions.md)表达式的允许结果之一。                                                                                                                                                                                                                                                                                                                                                                    |
| `PRUNE`   | [$redact](/aggregation/aggregation-reference/variables-in-aggregation-expressions.md)表达式的允许结果之一。                                                                                                                                                                                                                                                                                                                                                                    |
| `KEEP`    | [$redact](/aggregation/aggregation-reference/variables-in-aggregation-expressions.md)表达式的允许结果之一。                                                                                                                                                                                                                                                                                                                                                                    |

> **也可以看看**
>
> [$let](/aggregation/aggregation-reference/variables-in-aggregation-expressions.md)，[$redact](/aggregation/aggregation-reference/variables-in-aggregation-expressions.md)，[$map](/aggregation/aggregation-reference/variables-in-aggregation-expressions.md)

译者：李冠飞

校对：


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