你想要的是能够看到 NULL 文档的值，这在默认情况下是不可能的 NULL 数值不可索引和搜索，更多信息请参考官方的空值文档
无法索引或搜索空值。当一个字段设置为null（或空数组或空值数组）时，它被视为该字段没有值。
但是为了实现您的用例，您需要提供 null_value 在Map和索引文档时，如果没有收到值，则传递 null_value 文件中的param值，如下面的完整示例所示
索引Map

{
    "mappings": {
        "properties": {
            "accidents": {
                "type": "keyword",
                "null_value": "NULL" --> note this
            },
            "accidents_date": {
                "type": "keyword",
                "null_value": "NULL"
            },
            "car_registration_no" :{
                "type" : "keyword"
            }
        }
    }
}

索引示例文档

{
    "accidents" : "1",
    "accidents_date": "12/12/2019",
    "car_registration_no" : "61124"
}
{
    "accidents" : "null",
    "accidents_date": "null",
    "car_registration_no" : "61125"
}
{
    "accidents" : "null",
    "accidents_date": "null",
    "car_registration_no" : "61123"
}

搜索查询

{
    "query": {
        "terms": {
            "car_registration_no": [
                "61124",
                "61123",
                "61125"
            ],
            "boost": 1.0
        }
    }
}

和预期的搜索结果

"hits": [
            {
                "_index": "carindex",
                "_type": "_doc",
                "_id": "1",
                "_score": 1.0,
                "_source": {
                    "accidents": "null", --> note this
                    "accidents_date": "NULL",
                    "car_registration_no": "61123"
                }
            },
            {
                "_index": "carindex",
                "_type": "_doc",
                "_id": "2",
                "_score": 1.0,
                "_source": {
                    "accidents": "1",
                    "accidents_date": "12/12/2019",
                    "car_registration_no": "61124"
                }
            },
            {
                "_index": "carindex",
                "_type": "_doc",
                "_id": "3",
                "_score": 1.0,
                "_source": {
                    "car_registration_no": "61125"
                }
            }
        ]

可以使用术语查询返回在提供的字段中包含一个或多个精确术语的文档。
添加带有搜索查询和搜索结果的工作示例（使用与问题中提供的相同的示例索引数据）
搜索查询：（版本7.*）

{
  "query": {
    "terms": {
      "row.primaryKey.car_registration_no": [ "61125","61123", "61124" ]
    }
  }
}

搜索结果：

{
  "took": 16,
  "timed_out": false,
  "_shards": {
    "total": 1,
    "successful": 1,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": {
      "value": 3,
      "relation": "eq"
    },
    "max_score": 1.0,
    "hits": [
      {
        "_index": "stof_64398592",
        "_type": "_doc",
        "_id": "1",
        "_score": 1.0,
        "_source": {
          "schemaid": "adef89sesdceasjkmn5dlky6djj7kk189lkdqsc",
          "localVersion": 2,
          "row": {
            "primaryKey": {
              "car_registration_no": "61123"
            },
            "columns": {
              "model": "Nissan",
              "submodel": "Saloon",
              "accidents": "null",
              "accident_date": "null",         <-- note this
              "date_used": 1597017600000
            },
            "editsVersion": 0
          },
          "highlight": {}
        }
      },
      {
        "_index": "stof_64398592",
        "_type": "_doc",
        "_id": "2",
        "_score": 1.0,
        "_source": {
          "schemaid": "adef89sesdceasjkmn5dlky6djj7kk189lkdqsc",
          "localVersion": 2,
          "row": {
            "primaryKey": {
              "car_registration_no": "61124"
            },
            "columns": {
              "model": "Nissan",
              "submodel": "Saloon",
              "accidents": "1",
              "accident_date": "12/12/2019",
              "date_used": 1597017600000            <-- note this
            },
            "editsVersion": 0
          },
          "highlight": {}
        }
      },
      {
        "_index": "stof_64398592",
        "_type": "_doc",
        "_id": "3",
        "_score": 1.0,
        "_source": {
          "schemaid": "adef89sesdceasjkmn5dlky6djj7kk189lkdqsc",
          "localVersion": 2,
          "row": {
            "primaryKey": {
              "car_registration_no": "61125"
            },
            "columns": {
              "model": "Nissan",
              "submodel": "Saloon",
              "accidents": "null",
              "accident_date": "null",           <-- note this
              "date_used": 1597017600000
            },
            "editsVersion": 0
          },
          "highlight": {}
        }
      }
    ]
  }
}

如果您使用的是版本5.*，那么请阅读有关术语查询的文档，了解其语法。