Hey，大家好！我是 Elasticsearch 新手。我想知道如何通过 Query DSL 找出指定字段值为 null 的文档。
以下是我索引（索引名：class_info_v22002，用来存放班级信息）创建的 DSL：
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| PUT class_info_v22002
{
"settings": {
"refresh_interval": "1s",
"number_of_replicas": 1,
"number_of_shards": 3
},
"mappings": {
"properties": {
"class_id": {
"type": "integer"
},
"class_name": {
"type": "keyword"
},
"teacher": {
"type": "keyword"
}
}
}
}
|
索引的文档内容类似如下的示例数据（通过批量添加数据模拟）：
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| PUT class_info_v22002/_bulk
{"index": {"_id": "1"}}
{"class_id": 2008001,"class_name": "三年（1）班","teacher": "张三"}
{"index": {"_id": "2"}}
{"class_id": 2008002,"class_name": "三年（2）班","teacher": "李四"}
{"index": {"_id": "3"}}
{"class_id": 2008003,"class_name": "三年（3）班","teacher": "王五"}
{"index": {"_id": "4"}}
{"class_id": 2008004,"class_name": "三年（4）班","teacher": null}
|
如果要找出 teacher 字段值为 null 的文档，Query DSL 应该如何写？
你好，你可以使用以下的 DSL 筛选出 teacher 字段值为 null 的文档：
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| GET class_info_v22002/_search
{
"query": {
"bool": {
"must_not": {
"exists": {
"field": "teacher"
}
}
}
}
}
|
返回结果如下：
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| {
"took" : 1,
"timed_out" : false,
"_shards" : {
"total" : 5,
"successful" : 5,
"skipped" : 0,
"failed" : 0
},
"hits" : {
"total" : {
"value" : 1,
"relation" : "eq"
},
"max_score" : 0.0,
"hits" : [
{
"_index" : "class_info_v22002",
"_type" : "_doc",
"_id" : "4",
"_score" : 0.0,
"_source" : {
"class_id" : 2008004,
"class_name" : "三年（4）班",
"teacher" : null
}
}
]
}
}
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你也可以使用以下 SQL 来实现：
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| POST _sql?format=txt
{
"query": "select * from class_info_v22002 where teacher is null"
}
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返回结果：
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| class_id | class_name | teacher
---------------+---------------+---------------
2008004 |三年（4）班 |null
|
（END）
