# Dictionary

A dictionary is a mapping (key -> attributes) that can be used as a function for queries, offering a simpler and more efficient approach compared to the `JOIN` method of reference tables.

There are two types of data dictionaries: built-in dictionaries and external dictionaries.

### Built-in Dictionary

ClickHouse supports a [built-in dictionary](https://clickhouse.tech/docs/en/query_language/dicts/internal_dicts/) called geobase. The supported functions can be referenced in [Functions for working with Yandex.Metrica dictionaries](https://clickhouse.tech/docs/en/query_language/functions/ym_dict_functions/).

### External Dictionary

ClickHouse can add [external dictionaries](https://clickhouse.tech/docs/en/query_language/dicts/external_dicts/) from multiple data sources. Supported data sources can be found at [Sources Of External Dictionaries](https://clickhouse.tech/docs/en/query_language/dicts/external_dicts_dict_sources/).