> ## Documentation Index
> Fetch the complete documentation index at: https://private-7c7dfe99-mintlify-3a82795f.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

> Overview of domain types in ClickHouse, which extend base types with additional features

# Domains

Domains are special-purpose types that add extra features on top of existing base types, while leaving the on-wire and on-disk format of the underlying data type intact. Currently, ClickHouse does not support user-defined domains.

You can use domains anywhere corresponding base type can be used, for example:

* Create a column of a domain type
* Read/write values from/to domain column
* Use it as an index if a base type can be used as an index
* Call functions with values of domain column

<h3 id="extra-features-of-domains">
  Extra Features of Domains
</h3>

* Explicit column type name in `SHOW CREATE TABLE` or `DESCRIBE TABLE`
* Input from human-friendly format with `INSERT INTO domain_table(domain_column) VALUES(...)`
* Output to human-friendly format for `SELECT domain_column FROM domain_table`
* Loading data from an external source in the human-friendly format: `INSERT INTO domain_table FORMAT CSV ...`

<h3 id="limitations">
  Limitations
</h3>

* Can't convert index column of base type to domain type via `ALTER TABLE`.
* Can't implicitly convert string values into domain values when inserting data from another column or table.
* Domain adds no constrains on stored values.