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112 lines
3.1 KiB
Markdown
112 lines
3.1 KiB
Markdown
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# Dictionary
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`Dictionary` 引擎将字典数据展示为一个ClickHouse的表。
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例如，考虑使用一个具有以下配置的 `products` 字典：
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```xml
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<dictionaries>
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<dictionary>
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<name>products</name>
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<source>
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<odbc>
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<table>products</table>
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<connection_string>DSN=some-db-server</connection_string>
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</odbc>
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</source>
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<lifetime>
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<min>300</min>
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<max>360</max>
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</lifetime>
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<layout>
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<flat/>
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</layout>
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<structure>
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<id>
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<name>product_id</name>
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</id>
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<attribute>
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<name>title</name>
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<type>String</type>
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<null_value></null_value>
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</attribute>
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</structure>
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</dictionary>
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</dictionaries>
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```
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查询字典中的数据：
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``` sql
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select name, type, key, attribute.names, attribute.types, bytes_allocated, element_count,source from system.dictionaries where name = 'products';
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SELECT
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name,
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type,
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key,
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attribute.names,
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attribute.types,
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bytes_allocated,
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element_count,
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source
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FROM system.dictionaries
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WHERE name = 'products'
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```
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```
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┌─name─────┬─type─┬─key────┬─attribute.names─┬─attribute.types─┬─bytes_allocated─┬─element_count─┬─source──────────┐
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│ products │ Flat │ UInt64 │ ['title'] │ ['String'] │ 23065376 │ 175032 │ ODBC: .products │
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└──────────┴──────┴────────┴─────────────────┴─────────────────┴─────────────────┴───────────────┴─────────────────┘
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```
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你可以使用 [dictGet*](../../query_language/functions/ext_dict_functions.md) 函数来获取这种格式的字典数据。
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当你需要获取原始数据，或者是想要使用 `JOIN` 操作的时候，这种视图并没有什么帮助。对于这些情况，你可以使用 `Dictionary` 引擎，它可以将字典数据展示在表中。
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语法：
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```
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CREATE TABLE %table_name% (%fields%) engine = Dictionary(%dictionary_name%)`
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```
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示例：
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``` sql
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create table products (product_id UInt64, title String) Engine = Dictionary(products);
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CREATE TABLE products
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(
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product_id UInt64,
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title String,
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)
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ENGINE = Dictionary(products)
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```
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```
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Ok.
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0 rows in set. Elapsed: 0.004 sec.
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```
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看一看表中的内容。
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``` sql
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select * from products limit 1;
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SELECT *
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FROM products
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LIMIT 1
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```
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```
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┌────product_id─┬─title───────────┐
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│ 152689 │ Some item │
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└───────────────┴─────────────────┘
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1 rows in set. Elapsed: 0.006 sec.
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```
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[来源文章](https://clickhouse.yandex/docs/en/operations/table_engines/dictionary/) <!--hide-->
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