原文地址: https://www.elastic.co/guide/en/elasticsearch/reference/7.7/null-value.html, 原文档版权归 www.elastic.co 所有
IMPORTANT: No additional bug fixes or documentation updates will be released for this version. For the latest information, see the current release documentation.
Elasticsearch Guide [7.7] » Mapping » Mapping parameters » null_value
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null_valueedit

A null value cannot be indexed or searched. When a field is set to null, (or an empty array or an array of null values) it is treated as though that field has no values.

The null_value parameter allows you to replace explicit null values with the specified value so that it can be indexed and searched. For instance:

PUT my_index
{
  "mappings": {
    "properties": {
      "status_code": {
        "type":       "keyword",
        "null_value": "NULL" 
      }
    }
  }
}

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

PUT my_index/_doc/2
{
  "status_code": [] 
}

GET my_index/_search
{
  "query": {
    "term": {
      "status_code": "NULL" 
    }
  }
}

Replace explicit null values with the term NULL.

An empty array does not contain an explicit null, and so won’t be replaced with the null_value.

A query for NULL returns document 1, but not document 2.

The null_value needs to be the same datatype as the field. For instance, a long field cannot have a string null_value.

The null_value only influences how data is indexed, it doesn’t modify the _source document.

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