# null\_value（空值）

**null** 值不能被索引或者搜索.当一个字段被设置成 **null**（或者一个空数组,或者值全为 **null** 的数组）时， 该字段将被视为没有值。

**null\_value** 参数允许你用一个特殊的值替换一个显示的 **null** 值， 以确保这个字段能被索引和搜索。例如：

```
curl -XPUT 'localhost:9200/my_index?pretty' -H 'Content-Type: application/json' -d'
{
  "mappings": {
    "my_type": {
      "properties": {
        "status_code": {
          "type":       "keyword",
          "null_value": "NULL" #1
        }
      }
    }
  }
}
'
curl -XPUT 'localhost:9200/my_index/my_type/1?pretty' -H 'Content-Type: application/json' -d'
{
  "status_code": null
}
'
curl -XPUT 'localhost:9200/my_index/my_type/2?pretty' -H 'Content-Type: application/json' -d'
{
  "status_code": [] #2
}
'
curl -XGET 'localhost:9200/my_index/_search?pretty' -H 'Content-Type: application/json' -d'
{
  "query": {
    "term": {
      "status_code": "NULL" #3
    }
  }
}
'
```

| 1 | 用**NULL**代替显示的**null**值。                  |
| - | ----------------------------------------- |
| 2 | 没有显示的包含**null**的空数组，不会被**null\_value**替换。 |
| 3 | 搜索**NULL**将返回文档1，而不会返回文档2。                |

> 重点
>
> **null\_value**必须设置成相同类型的参数。例如，一个**long**型的字段不能被设置成**string**类型的**null\_value**。

## 解析:

null值比较特殊,传统的方式,无法搜索会报错

如下

```
PUT /my_index
{
  "mappings": {
    "my_type": {
      "properties": {
        "status_code": {
          "type": "keyword"
        }
      }
    }
  }
}

PUT /my_index/my_type/1
{
  "status_code": null
}

GET /my_index/_search
{
  "query": {
    "term": {
      "status_code": null
    }
  }
}
```

查询报错

```
{
  "error": {
    "root_cause": [
      {
        "type": "illegal_argument_exception",
        "reason": "field name is null or empty"
      }
    ],
    "type": "illegal_argument_exception",
    "reason": "field name is null or empty"
  },
  "status": 400
}
```

所以为了解决这个问题,采用一个值来替换null值的情况. 如开始时候的例子,采用"NULL" 字符串采替换null值的情况进行搜索.


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