SqlServer存储过程转换为MySQL存储过程

求帮忙将SqlServer存储过程转换为MySQL存储过程，咨询了很多人都不会：

-- 存储过程：登陆
-- 返回结果success和message：1表成功 0 表失败，message是提示消息

-- 执行下句测试，会显示1，登陆成功。
-- exec proc_login '1','1';

create proc proc_login
@username varchar(20),
@password varchar(20)
as begin
if exists(select * from tb_admin where @username=username)
begin
if exists(select * from tb_admin where @username=username and @password=password)
select 1 as success,'登陆成功' as message;
else
select 0 as success,'密码错误' as message;
end
else
begin
select 0 as success,'用户名不存在' as message;
end
end
go

delimiter $$
create procedure proc_login (in un varchar(20),in pw varchar(20))
begin
    if exists (select * from tb_admin where username=un)
    then
        if exists (select * from tb_admin where username=un and password=pw)
        then select 1 as success,'登陆成功' as message;
        else
        select 0 as success,'密码错误' as message;
        end if;
    else 
    select 0 as success,'用户名不存在' as message;
    end if;
end $$
delimiter ;

call proc_login('1','1')