In this tutorial, you will learn how to create stored functions using CREATE FUNCTION statement.

A stored function is a special kind stored program that returns a single value. You use stored functions to encapsulate common formulas or business rules that may be reusable among SQL statements or stored programs.

Different from a stored procedure, you can use a stored function in SQL statements wherever an expression is used. This helps improve the readability and maintainability of the procedural code.

MySQL stored function syntax

The following illustrates the simplest syntax for creating a new stored function:

CREATE FUNCTION function_name(param1,param2,…)     RETURNS datatype    [NOT] DETERMINISTIC  statements

First, you specify the name of the stored function after  CREATE FUNCTION keywords.

Second, you list all parameters of the stored function. By default, all parameters are implicitly IN parameters. You cannot specify IN, OUT or INOUT modifiers to the parameters.

Third, you must specify the data type of the return value in the RETURNS statement. It can be any valid MySQL data types.

Fourth, for the same input parameters, if the stored function returns the same result, it is considered deterministic and not deterministic otherwise.

You have to decide whether a stored function is deterministic or not. If you declare it incorrectly, the stored function may produced an unexpected result, or the available optimization is not used which degrade the performance.

Fifth, you write the code in the statements section. It can be a single statement or a compound statement. Inside the statements section, you have to specify at least one RETURN statement.

The RETURN statement returns a value to the caller. Whenever the RETURN statement is reached, the stored function’s execution is terminated immediately.

MySQL stored function example

The following example is a function that returns level of customer based on credit limit.

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 can call the CustomerLevel() in an SQL SELECT statement as follows:

SELECT customerName, CustomerLevel(creditLimit) FROM customers;

We also rewrite the  GetCustomerLevel() stored procedure that we developed in the MySQL IF statement tutorial as follows:

DELIMITER $$ CREATE PROCEDURE GetCustomerLevel(     IN  p_customerNumber INT(11),     OUT p_customerLevel  varchar(10) ) BEGIN     DECLARE creditlim DOUBLE;     SELECT creditlimit INTO creditlim     FROM customers     WHERE customerNumber = p_customerNumber;     SELECT CUSTOMERLEVEL(creditlim)      INTO p_customerLevel; END

As you can see, the  GetCustomerLevel() stored procedure is much more readable when using the  CustomerLevel() stored function.

Notice that a stored function returns a single value only. If you include a SELECT statement without INTO clause, you will get an error.

In addition, if a stored function contains SQL statements, you should not use it inside other SQL statements; otherwise the stored function will cause the performance of the SQL statements to degrade.

原文链接：http://outofmemory.cn/mysql/procedure/mysql-stored-function