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3.1 KiB
3.1 KiB
Dictionary
Dictionary 引擎将字典数据展示为一个ClickHouse的表。
例如，考虑使用一个具有以下配置的 products 字典：
<dictionaries>
<dictionary>
<name>products</name>
<source>
<odbc>
<table>products</table>
<connection_string>DSN=some-db-server</connection_string>
</odbc>
</source>
<lifetime>
<min>300</min>
<max>360</max>
</lifetime>
<layout>
<flat/>
</layout>
<structure>
<id>
<name>product_id</name>
</id>
<attribute>
<name>title</name>
<type>String</type>
<null_value></null_value>
</attribute>
</structure>
</dictionary>
</dictionaries>
查询字典中的数据：
select name, type, key, attribute.names, attribute.types, bytes_allocated, element_count,source from system.dictionaries where name = 'products';
SELECT
name,
type,
key,
attribute.names,
attribute.types,
bytes_allocated,
element_count,
source
FROM system.dictionaries
WHERE name = 'products'
┌─name─────┬─type─┬─key────┬─attribute.names─┬─attribute.types─┬─bytes_allocated─┬─element_count─┬─source──────────┐
│ products │ Flat │ UInt64 │ ['title'] │ ['String'] │ 23065376 │ 175032 │ ODBC: .products │
└──────────┴──────┴────────┴─────────────────┴─────────────────┴─────────────────┴───────────────┴─────────────────┘
你可以使用 dictGet* 函数来获取这种格式的字典数据。
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。
语法：
CREATE TABLE %table_name% (%fields%) engine = Dictionary(%dictionary_name%)`
示例：
create table products (product_id UInt64, title String) Engine = Dictionary(products);
CREATE TABLE products
(
product_id UInt64,
title String,
)
ENGINE = Dictionary(products)
Ok.
0 rows in set. Elapsed: 0.004 sec.
看一看表中的内容。
select * from products limit 1;
SELECT *
FROM products
LIMIT 1
┌────product_id─┬─title───────────┐
│ 152689 │ Some item │
└───────────────┴─────────────────┘
1 rows in set. Elapsed: 0.006 sec.