How to Create UDFs in MySQL

How do I create User-Defined Functions (UDFs) in MySQL?

MySQL UDFs (User-Defined Functions) let you encapsulate reusable logic that can be called in SQL, similar to built-in functions.

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Description

What is a UDF in MySQL?

A User-Defined Function (UDF) is a piece of reusable SQL or external code registered in MySQL and callable like SUM() or NOW(). UDFs help centralize business logic, improve readability, and reduce repetition.

When should I create a UDF?

Create a UDF when the same calculation appears in multiple queries, when complex logic clutters SELECT lists, or when enforcing consistent business rules across reports.

How do I create a SQL UDF in MySQL?

Use CREATE FUNCTION with a deterministic body that returns a single value. Store the function in the chosen database schema so any user with EXECUTE privilege can call it.

Step-by-step example

1. Switch to the target schema.
2. Write the CREATE FUNCTION statement.
3. Grant privileges if others need to execute it.

Practical ecommerce use case

Suppose every order requires a 2% marketplace fee. Instead of repeating total_amount * 0.02, wrap it in fn_marketplace_fee(). Analysts and services now call the function, guaranteeing identical logic.

Best practices for MySQL UDFs

• Mark functions DETERMINISTIC when they always return the same output for the same input.
• Keep functions side-effect-free—avoid INSERT/UPDATE.
• Version functions with clear names (fn_ prefix) to avoid collisions.
• Document parameter types and units in comments.

How to alter or drop a UDF?

MySQL lacks ALTER FUNCTION; instead, drop the function and recreate it. Use DROP FUNCTION IF EXISTS fn_name; to avoid errors in deployment scripts.

How do I grant execute rights?

Run GRANT EXECUTE ON FUNCTION db.fn_name TO 'analyst'@'%';. Revoke with REVOKE EXECUTE ON FUNCTION db.fn_name FROM 'analyst'@'%';.

Why How to Create UDFs in MySQL is important

How to Create UDFs in MySQL Example Usage


-- Calculate net revenue after marketplace fee for each order
SELECT o.id,
       o.total_amount,
       fn_marketplace_fee(o.total_amount)       AS fee,
       o.total_amount - fn_marketplace_fee(o.total_amount) AS net_revenue
FROM Orders AS o
ORDER BY o.order_date DESC;

How to Create UDFs in MySQL Syntax


CREATE FUNCTION fn_marketplace_fee(order_total DECIMAL(10,2))
RETURNS DECIMAL(10,2)
DETERMINISTIC
BEGIN
    RETURN order_total * 0.02;
END;

-- Usage in queries
SELECT id, fn_marketplace_fee(total_amount) AS fee
FROM Orders;

Common Mistakes

Omitting DETERMINISTIC/NONDETERMINISTIC. Why wrong: MySQL can’t optimize function calls and may produce incorrect replication with statement-based logging. Fix: Declare DETERMINISTIC if the function always returns the same output; otherwise declare NONDETERMINISTIC.
Using UDFs for data-modifying logic. Why wrong: Functions should return a value, not change data. Side effects break expectations and can silently fail. Fix: Move INSERT/UPDATE logic into stored procedures or application code.

Frequently Asked Questions (FAQs)

Can a UDF return JSON?

Yes. Define the return type as JSON and build the object with JSON_OBJECT() inside the function body.

Do UDFs slow down queries?

Scalar UDFs introduce minimal overhead, but complex calculations in large result sets can add latency. Mark functions DETERMINISTIC and avoid unnecessary calls.