User-Defined Functions (UDFs) vs Stored Procedures in MySQL
User-defined functions (UDFs) and stored procedures are both reusable database routines in MySQL, but they differ in purpose, behavior, and how they are used in queries.
Return a single value (scalar) every time they are called.
Can be used inside SQL expressions (e.g., SELECT, WHERE, ORDER BY).
Must not modify data (no INSERT, UPDATE, DELETE, or transactions).
Must return a value using RETURN.
Are deterministic or non-deterministic, depending on their logic.
Perform a sequence of operations—often complex logic.
Can return zero, one, or many values (via OUT parameters or result sets).
Can modify data (INSERT, UPDATE, DELETE allowed).
Cannot be used inside SELECT statements.
Support control flow, loops, and exception handling.
UDFs return a single value; procedures can return multiple values.
UDFs cannot change database state; procedures can.
UDFs are called inside SQL queries; procedures are invoked with CALL.
UDFs must be deterministic to be optimized; procedures do not require this.
Procedures support complex logic, loops, and error handling; UDFs do not.
In summary: Use UDFs for computations within queries that return a value, and use stored procedures for performing operations, workflows, or multi-step logic on the database.