[[null-value]]
=== `null_value`

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:

[source,console]
--------------------------------------------------
PUT my_index
{
  "mappings": {
    "properties": {
      "status_code": {
        "type":       "keyword",
        "null_value": "NULL" <1>
      }
    }
  }
}

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

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

GET my_index/_search
{
  "query": {
    "term": {
      "status_code": "NULL" <3>
    }
  }
}
--------------------------------------------------

<1> Replace explicit `null` values with the term `NULL`.
<2> An empty array does not contain an explicit `null`, and so won't be replaced with the `null_value`.
<3> A query for `NULL` returns document 1, but not document 2.

IMPORTANT: The `null_value` needs to be the same data type as the field.  For
instance, a `long` field cannot have a string `null_value`.

NOTE: The `null_value` only influences how data is indexed, it doesn't modify
the `_source` document.