MySQL custom function

Preface

MySQL It provides built-in functions , The existence of these functions brings great convenience to our daily development and data operation , Like the aggregate function I mentioned earlier SUM()、AVG() And date time functions and so on , But we always have other needs ： We need to call a function , This function needs to implement our own functions according to our requirements , However, this functional system cannot provide because of the uncertainty of demand . So we need to solve this demand by ourselves . not so bad MySQL The extensibility of the design gives us this opportunity , We can solve this problem through the function of custom function .

brief introduction

stay MySQL The use of custom functions in also requires corresponding requirements , The grammar is as follows ,

Create a new function ：

　　　　Create function function_name( parameter list )

　　　　returns return type 

　　　　 Function body content 

Related instructions

• Function name ： Should be a legal identifier , And should not conflict with existing keywords . A function should belong to a database , have access to db_name.funciton_name Execute the database to which the current function belongs
  　　　　, Otherwise, it defaults to the current database .

• parameter list ： You can have one or more function parameters , Even without parameters . For each parameter , It consists of parameter name and parameter type .

• Return value ： Indicates the return value class type

• The body of the function ： The function body of a custom function consists of several available MySQL sentence , Process control , Variable declaration and other statements . It should be noted that the function body must contain return Return statement .

Examples of custom functions

[1] Functions without parameters

DELIMITER $$
create function say_hello()
RETURNS VARCHAR(255)
BEGIN
	return 'Ha HA HA HA HA Hello!!!!';
END $$
DELIMITER ;

Call function ：

select say_hello();

select say_hello(),b.* from book b;

[2] Functions with parameters

DELIMITER $$
create function format_title(title varchar(255))
RETURNS VARCHAR(255)
BEGIN
	declare v_title varchar(255);
	if length(title)>5 THEN
		select CONCAT(SUBSTR(title,1,5),'...') into v_title;
	ELSE
		set v_title = title;
	end if;
	return v_title;
END $$
DELIMITER ;

Call functions with parameters ：

select format_title(title) from book