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14 lines
729 B
Markdown
14 lines
729 B
Markdown
# 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-literal) 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#ym_dict_functions).
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- [Plug-in (external) dictionaries](external_dicts.md#dicts-external_dicts) with a [set of functions](../functions/ext_dict_functions.md#ext_dict_functions).
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