ClickHouse/docs/en/query_language/dicts/index.md

# Dictionaries

A dictionary is a mapping (`key -> attributes`) that is convenient for various types of reference lists.

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.

[NULL](../syntax.md#null-literal) values can't be stored in a dictionary.

ClickHouse supports:

- [Built-in dictionaries](internal_dicts.md#internal_dicts) with a specific [set of functions](../functions/ym_dict_functions.md#ym_dict_functions).
- [Plug-in (external) dictionaries](external_dicts.md#dicts-external_dicts) with a [set of functions](../functions/ext_dict_functions.md#ext_dict_functions).
Sources for english documentation switched to Markdown. Edit page link is fixed too for both language versions of documentation. 2017-12-28 15:13:23 +00:00			`# Dictionaries`

Update of english documentation (#2918) * Updating of english translation. * Some bugs are fixed. 2018-09-04 11:18:59 +00:00			A dictionary is a mapping (`key -> attributes`) that is convenient for various types of reference lists.
Sources for english documentation switched to Markdown. Edit page link is fixed too for both language versions of documentation. 2017-12-28 15:13:23 +00:00
Update of english documentation (#2918) * Updating of english translation. * Some bugs are fixed. 2018-09-04 11:18:59 +00:00			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.

			`[NULL](../syntax.md#null-literal) values can't be stored in a dictionary.`

			`ClickHouse supports:`

			`- [Built-in dictionaries](internal_dicts.md#internal_dicts) with a specific [set of functions](../functions/ym_dict_functions.md#ym_dict_functions).`
			`- [Plug-in (external) dictionaries](external_dicts.md#dicts-external_dicts) with a [set of functions](../functions/ext_dict_functions.md#ext_dict_functions).`
Sources for english documentation switched to Markdown. Edit page link is fixed too for both language versions of documentation. 2017-12-28 15:13:23 +00:00