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).


[Original article](https://clickhouse.yandex/docs/en/query_language/dicts/) <!--hide-->
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
WIP on docs/website (#3383) * CLICKHOUSE-4063: less manual html @ index.md * CLICKHOUSE-4063: recommend markdown="1" in README.md * CLICKHOUSE-4003: manually purge custom.css for now * CLICKHOUSE-4064: expand <details> before any print (including to pdf) * CLICKHOUSE-3927: rearrange interfaces/formats.md a bit * CLICKHOUSE-3306: add few http headers * Remove copy-paste introduced in #3392 * Hopefully better chinese fonts #3392 * get rid of tabs @ custom.css * Apply comments and patch from #3384 * Add jdbc.md to ToC and some translation, though it still looks badly incomplete * minor punctuation * Add some backlinks to official website from mirrors that just blindly take markdown sources * Do not make fonts extra light * find . -name '.md' -type f \| xargs -I{} perl -pi -e 's//g' {} find . -name '.md' -type f \| xargs -I{} perl -pi -e 's/ sql/g' {} Remove outdated stuff from roadmap.md * Not so light font on front page too * Refactor Chinese formats.md to match recent changes in other languages 2018-10-16 10:47:17 +00:00
			`[Original article](https://clickhouse.yandex/docs/en/query_language/dicts/) <!--hide-->`