ClickHouse/docs/en/sql_reference/dictionaries/index.md

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Dictionaries	35	Introduction

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 values can’t be stored in a dictionary.

ClickHouse supports:

• Built-in dictionaries with a specific set of functions.
• Plug-in (external) dictionaries with a net of functions.

Original article