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21 lines
908 B
Markdown
21 lines
908 B
Markdown
---
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toc_folder_title: Dictionaries
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toc_priority: 35
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toc_title: Introduction
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---
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# Dictionaries {#dictionaries}
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A dictionary is a mapping (`key -> attributes`) that is convenient for various types of reference lists.
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ClickHouse supports special functions for working with dictionaries that can be used in queries. It is easier and more efficient to use dictionaries with functions than a `JOIN` with reference tables.
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[NULL](../syntax.md#null) values can’t be stored in a dictionary.
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ClickHouse supports:
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- [Built-in dictionaries](internal_dicts.md#internal_dicts) with a specific [set of functions](../../sql_reference/functions/ym_dict_functions.md).
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- [Plug-in (external) dictionaries](external_dictionaries/external_dicts.md) with a [net of functions](../../sql_reference/functions/ext_dict_functions.md).
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[Original article](https://clickhouse.tech/docs/en/query_language/dicts/) <!--hide-->
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