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105 lines
3.2 KiB
Markdown
105 lines
3.2 KiB
Markdown
---
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machine_translated: true
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machine_translated_rev: 72537a2d527c63c07aa5d2361a8829f3895cf2bd
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toc_priority: 45
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toc_title: hdfs
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---
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# hdfs {#hdfs}
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从HDFS中的文件创建表。 此表函数类似于 [url](url.md) 和 [文件](file.md) 一些的。
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``` sql
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hdfs(URI, format, structure)
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```
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**输入参数**
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- `URI` — The relative URI to the file in HDFS. Path to file support following globs in readonly mode: `*`, `?`, `{abc,def}` 和 `{N..M}` 哪里 `N`, `M` — numbers, \``'abc', 'def'` — strings.
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- `format` — The [格式](../../interfaces/formats.md#formats) 的文件。
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- `structure` — Structure of the table. Format `'column1_name column1_type, column2_name column2_type, ...'`.
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**返回值**
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具有指定结构的表，用于读取或写入指定文件中的数据。
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**示例**
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表从 `hdfs://hdfs1:9000/test` 并从中选择前两行:
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``` sql
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SELECT *
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FROM hdfs('hdfs://hdfs1:9000/test', 'TSV', 'column1 UInt32, column2 UInt32, column3 UInt32')
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LIMIT 2
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```
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``` text
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┌─column1─┬─column2─┬─column3─┐
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│ 1 │ 2 │ 3 │
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│ 3 │ 2 │ 1 │
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└─────────┴─────────┴─────────┘
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```
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**路径中的水珠**
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多个路径组件可以具有globs。 对于正在处理的文件应该存在并匹配到整个路径模式（不仅后缀或前缀）。
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- `*` — Substitutes any number of any characters except `/` 包括空字符串。
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- `?` — Substitutes any single character.
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- `{some_string,another_string,yet_another_one}` — Substitutes any of strings `'some_string', 'another_string', 'yet_another_one'`.
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- `{N..M}` — Substitutes any number in range from N to M including both borders.
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建筑与 `{}` 类似于 [远程表功能](../../sql-reference/table-functions/remote.md)).
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**示例**
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1. 假设我们在HDFS上有几个具有以下Uri的文件:
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- ‘hdfs://hdfs1:9000/some_dir/some_file_1’
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- ‘hdfs://hdfs1:9000/some_dir/some_file_2’
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- ‘hdfs://hdfs1:9000/some_dir/some_file_3’
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- ‘hdfs://hdfs1:9000/another_dir/some_file_1’
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- ‘hdfs://hdfs1:9000/another_dir/some_file_2’
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- ‘hdfs://hdfs1:9000/another_dir/some_file_3’
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1. 查询这些文件中的行数:
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<!-- -->
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``` sql
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SELECT count(*)
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FROM hdfs('hdfs://hdfs1:9000/{some,another}_dir/some_file_{1..3}', 'TSV', 'name String, value UInt32')
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```
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1. 查询这两个目录的所有文件中的行数:
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<!-- -->
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``` sql
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SELECT count(*)
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FROM hdfs('hdfs://hdfs1:9000/{some,another}_dir/*', 'TSV', 'name String, value UInt32')
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```
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!!! warning "警告"
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如果您的文件列表包含带前导零的数字范围，请单独使用带大括号的构造或使用 `?`.
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**示例**
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从名为 `file000`, `file001`, … , `file999`:
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``` sql
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SELECT count(*)
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FROM hdfs('hdfs://hdfs1:9000/big_dir/file{0..9}{0..9}{0..9}', 'CSV', 'name String, value UInt32')
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```
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## 虚拟列 {#virtual-columns}
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- `_path` — Path to the file.
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- `_file` — Name of the file.
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**另请参阅**
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- [虚拟列](https://clickhouse.tech/docs/en/operations/table_engines/#table_engines-virtual_columns)
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[原始文章](https://clickhouse.tech/docs/en/query_language/table_functions/hdfs/) <!--hide-->
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