2021-03-13 03:25:15 -05:00
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## 虚拟列(Virtual Columns)
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> [!WARNING]
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> Apache Druid支持两种查询语言： [Druid SQL](druidsql.md) 和 [原生查询](makeNativeQueries.md)。该文档描述了原生查询中的一种查询方式。 对于Druid SQL中使用的该种类型的信息，可以参考 [SQL文档](druidsql.md)。
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虚拟列是在查询期间从一组列创建的可查询列“视图”。
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一个虚拟列可能会从多个底层列中提取数据，尽管虚拟列总是以单个列的形式出现。
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虚拟列可以用作维度，也可以用作聚合器的输入。
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每个Apache Druid查询都可以接受一个虚拟列列表作为参数。提供以下扫描查询作为示例：
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```json
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{
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"queryType": "scan",
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"dataSource": "page_data",
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"columns":[],
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"virtualColumns": [
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{
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"type": "expression",
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"name": "fooPage",
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"expression": "concat('foo' + page)",
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"outputType": "STRING"
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},
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{
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"type": "expression",
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"name": "tripleWordCount",
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"expression": "wordCount * 3",
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"outputType": "LONG"
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}
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],
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"intervals": [
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"2013-01-01/2019-01-02"
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]
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}
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```
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### 虚拟列类型
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#### 表达式虚拟列
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表达式虚拟列语法如下：
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```json
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{
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"type": "expression",
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"name": <name of the virtual column>,
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"expression": <row expression>,
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"outputType": <output value type of expression>
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}
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```
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| 属性 | 描述 | 是否必须 |
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|-|-|-|
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| `name` | 虚拟列的名称 | 是 |
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| `expression` | 通过[表达式](expression.md)输入一些行，输出一个虚拟列的值 | 是 |
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| `outputType` | 表达式输出的类型，可能是 LONG、FLOAT、DOUBLE、STRING | 否（默认是FLOAT） |
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