MySQL stored functions - introduction

CREATE FUNCTION function_name(param1,param2,…)

    RETURNS datatype

   [NOT] DETERMINISTIC

statements

First you can define name of stored function after create function clause. 
List all parameters with in parenthesis. By default all parameters are IN parameters. You cannot define IN, INOUT, OUT modifiers to parameters.

You must specify the return data types in the return statements.

 Example: 

DELIMITER $$

CREATE FUNCTION CustomerLevel(p_creditLimit double) RETURNS VARCHAR(10)

    DETERMINISTIC

BEGIN

    DECLARE lvl varchar(10);

    IF p_creditLimit > 50000 THEN

SET lvl = 'PLATINUM';

    ELSEIF (p_creditLimit <= 50000 AND p_creditLimit >= 10000) THEN

        SET lvl = 'GOLD';

    ELSEIF p_creditLimit < 10000 THEN

        SET lvl = 'SILVER';

    END IF;

RETURN (lvl);

END

 Now we call CustomerLevel function in any sql function.

SELECT

    customerName, CustomerLevel(creditLimit)

FROM

    customers

ORDER BY customerName;

We can use this stored function with in stored procedures also.

MySQL stored functions - introduction