Doc filter nil op

internal/export/doc/en/datakit-filter.md

@@ -131,17 +131,22 @@ Support basic numerical comparison operations:
 { service = re('.*') AND ( abc IN [1,2,'foo', 2.3] OR def MATCH ['foo.*', 'bar.*']) }
 ```
 
+- Built-in constants
+
+    - `true/false`
+    - `nil/null`: null value, see following usage about them
+
 In addition, the following list operations are supported:
 
-| Operator              | Support Numeric Types   | Description                                                   | Example                              |
-| ----                | ----           | ----                                                   | ----                              |
-| `IN`, `NOTIN`       | Numeric list   | Whether the specified field is in a list, and multi-type cluttering is supported in the list           | `{ abc IN [1,2, "foo", 3.5]}`     |
+| Operator            | Support Numeric Types   | Description                                                                                   | Example                           |
+| ----                | ----                    | ----                                                                                          | ----                              |
+| `IN`, `NOTIN`       | Numeric list            | Whether the specified field is in a list, and multi-type cluttering is supported in the list  | `{ abc IN [1,2, "foo", 3.5]}`     |
 | `MATCH`, `NOTMATCH` | Regular expression list | Whether the specified field matches the regular in the list, which only supports string types | `{ abc MATCH ["foo.*", "bar.*"]}` |
 
 <!-- markdownlint-disable MD046 -->
 ???+ attention
 
-    **Only ordinary data types** such as string, integer, floating point can appear in the list. Other expressions are not supported.
+    - **Only ordinary data types** such as string, integer, floating point can appear in the list. Other expressions are not supported.
 
     The keywords `IN/NOTIN/MATCH/NOTMATCH` **are case insensitive**, meaning `in`, `IN` and `In` have the same effect. In addition, other operands are case sensitive, for example, the following filters express different meanings:
 
@@ -152,8 +157,34 @@ In addition, the following list operations are supported:
     ```
 
     In data point, all fields of **and their values are case-sensitive**.
+
+    - Identity/Non-identity Expression Syntax:
+
+    ```python
+    # Identity
+    { 1 == 1}
+    { 'abc' == 'abc'}
+
+    # Non-identity
+    { true != false }
+    { 'abc' != 'ABC' }  # String comparison is case-sensitive
+    ```
+
 <!-- markdownlint-enable -->
 
+### Usage of nil/null {#nil}
+
+`nil/null` is used to indicate the non-existence of certain fields, and can be used in `=/!=/IN/NOTIN` operations:
+
+```python
+{ some_tag_or_field != nil }                     # The field exists
+{ some_tag_or_field = nil }                      # The field does not exist
+{ some_tag_or_field IN ["abc", "123", null] }     # The field either does not exist or can only equal specified values
+{ some_tag_or_field NOTIN ["abc", "123", null] }  # The field is not null and not equal to specified values
+```
+
+Here, `nil/null` is not case sensitive and can be written in various forms such as `NULL/Null/NIL/Nil`.
+
 ## Usage Example {#usage}
 
 You can view the filtering using the `datakit monitor -V` command:

internal/export/doc/zh/datakit-filter.md

@@ -139,7 +139,12 @@ Filter 的主要功能就是数据筛选，其筛选依据是通过一定的筛
 { service = re('.*') AND ( abc IN [1,2,'foo', 2.3] OR def MATCH ['foo.*', 'bar.*']) }
 ```
 
-除此之外，还支持如下列表操作：
+- 内置常量
+
+    - `true/false`
+    - `nil/null`: 空值，见下文说明
+
+- 匹配和列表操作
 
 | 操作符              | 支持数值类型   | 说明                                                   | 示例                              |
 | ----                | ----           | ----                                                   | ----                              |
@@ -149,7 +154,7 @@ Filter 的主要功能就是数据筛选，其筛选依据是通过一定的筛
 <!-- markdownlint-disable MD046 -->
 ???+ attention
 
-    列表中**只能出现普通的数据类型**，如字符串、整数、浮点，其它表达式均不支持。 
+    - 列表中**只能出现普通的数据类型**，如字符串、整数、浮点，其它表达式均不支持。 
 
     `IN/NOTIN/MATCH/NOTMATCH` 这些关键字**大小写不敏感**，即 `in` 和 `IN` 以及 `In` 效果是一样的。除此之外，其它操作数的大小写都是敏感的，比如如下几个过滤器表达的意思不同：
 
@@ -160,8 +165,33 @@ Filter 的主要功能就是数据筛选，其筛选依据是通过一定的筛
     ```
 
     在数据点中，**所有字段以及其值都是大小写敏感的**。
+
+    - 恒等/恒不等表达式写法：
+
+    ```python
+    # 恒等
+    { 1 = 1}
+    { 'abc' = 'abc'}
+
+    # 恒不等
+    { true = false  }
+    { 'abc' = 'ABC' } # 字符串大小写敏感
+    ```
 <!-- markdownlint-enable -->
 
+### nil/null 用法 {#nil}
+
+`nil/null` 用于表示某些字段不存在，我们可以在 `=/!=/IN/NOTIN` 操作中使用：
+
+```python
+{ some_tag_or_field != nil }                     # 某个字段存在
+{ some_tag_or_field = nil }                      # 某个字段不存在
+{ some_tag_or_field IN ["abc", "123", null] }    # 某个字段要么不存在，要么只能等于指定的值
+{ some_tag_or_field NOTIN ["abc", "123", null] } # 某个字段不为 null 且不为指定的值
+```
+
+此处 `nil/null` 不区分大小写，我们可以写成 `NULL/Null/NIL/Nil` 等多种形式。
+
 ## 用法示例 {#usage}
 
 使用 `datakit monitor -V` 命令可以查看过滤情况：