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# 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 ](../functions/ym_dict_functions.md ).
- [Plug-in (external) dictionaries ](external_dicts.md ) with a [set of functions ](../functions/ext_dict_functions.md ).
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[Original article ](https://clickhouse.yandex/docs/en/query_language/dicts/ ) <!--hide-->