mysql，如何定义和使用数据库专用函数function？

delimiter $

delimiter ;

DROP TABLE IF EXISTS `posts`;
CREATE TABLE `posts` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `user_id` int(11) DEFAULT NULL,
  `title` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;
INSERT INTO `posts` VALUES (1,1,'1-苏南大叔的文章1'),(2,1,'1-苏南大叔的文章2'),(3,2,'2-苏南大哥的文章'),(4,9,'9-神秘人的文章');

DROP TABLE IF EXISTS `users`;
CREATE TABLE `users` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;
INSERT INTO `users` VALUES (1,'1-苏南大叔'),(2,'2-苏南大哥'),(3,'3-苏南兄弟');

create function 函数名（参数名1 参数类型1,参数名2 参数类型2,...） returns 返回类型
begin 
  函数体
End

no RETURN found

select 函数名(参数)

create function count_users() returns int
begin
    declare c int default 0;
    select count(*) into c from users;
    return c;
End

select count_users()
select count_users() as cnt

create function get_uid(userName VARCHAR(20)) returns int
begin
    set @uid=0;
    select id into @uid from users where name = userName;
    return @uid;
End

select get_uid("1-苏南大叔") as a,get_uid("苏南大叔") as b;

show create function 函数名

CREATE DEFINER=`root`@`localhost` FUNCTION `get_uid`(userName VARCHAR(20)) RETURNS int(11)
begin
set @uid=0;
select id into @uid from users where name = userName;
return @uid;
End;

drop function 函数名

drop function get_uid