Writing Custom ClickHouse® User-Defined Functions (UDFs)

When working with ClickHouse® User-Defined Functions (UDFs), you'll often encounter situations where the same expressions are repeated across multiple queries. Whether you're cleaning strings, standardizing calculations, or simplifying business logic, repeating complex expressions can make queries harder to maintain and understand.

ClickHouse® User-Defined Functions allow you to encapsulate reusable logic into a named function that can be called throughout your queries. Instead of copying and pasting lengthy expressions, you can define them once and reuse them wherever needed.

In this article, we'll explore how UDFs work in ClickHouse®, how to create them, common use cases, limitations, and practical examples that can help make your analytics workflows cleaner and easier to manage.

What Are User-Defined Functions?

A User-Defined Function (UDF) is a custom function that you create and register within ClickHouse®. Once created, it behaves similarly to a built-in function and can be referenced in SQL queries.

For example, imagine you frequently need to calculate the profit margin percentage:

(round((revenue - cost) / revenue * 100, 2))

Repeating this expression across dozens of dashboards and reports can become difficult to maintain. By creating a UDF, you can encapsulate the logic:

CREATE FUNCTION profit_margin AS
(revenue, cost) ->
round((revenue - cost) / revenue * 100, 2);

Now the same calculation becomes:

SELECT profit_margin(revenue, cost)
FROM sales;

This improves readability while ensuring consistent business logic across teams.

Why Use UDFs in ClickHouse®?

There are several reasons why UDFs can be useful in production environments.

Improved Query Readability

Complex calculations can quickly clutter SQL statements. Wrapping logic inside a descriptive function makes queries easier to understand.

Instead of:

SELECT
    round((revenue - cost) / revenue * 100, 2)
FROM sales;

You can write:

SELECT
    profit_margin(revenue, cost)
FROM sales;

The second version clearly communicates intent.

Reusable Business Logic

Organizations often have calculations that appear throughout dashboards, reports, and ETL pipelines.

Examples include:

• Profit calculations
• Currency conversions
• Data normalization
• String formatting
• Customer segmentation rules

Creating a UDF helps ensure everyone uses the same implementation.

Easier Maintenance

If business requirements change, you only need to update the function definition instead of modifying dozens of separate queries.

This reduces the risk of inconsistencies and human error.

Creating Your First ClickHouse® UDF

Let's create a simple function that converts a string to title case.

CREATE FUNCTION format_name AS
(name) -> initcap(lower(name));

Usage:

SELECT format_name('CLICKHOUSE');

Output:

Clickhouse

You can verify that the function exists using:

SHOW FUNCTIONS;

Or query the system catalog:

SELECT *
FROM system.functions
WHERE name = 'format_name';

This allows administrators and developers to inspect available functions within the environment.

Practical Examples

Example 1: Standardizing Email Domains

Suppose user records contain inconsistent capitalization.

CREATE FUNCTION normalize_email AS
(email) ->
lower(email);

Usage: