mysql存储过程返回值null

参数IN cid INT,OUT t INT
内容
BEGIN
DECLARE OFF INT default 0;
UPDATe competition c SET c.stat = c.stat + 1 WHERe c.id = cid;
SELECT c.stat INTO OFF FROM competition c WHERe c.id = cid;
SELECt m_time into t FROM module WHERe module.competition_id = cid LIMIT OFF,1;
SELECT OFF,t;
END
返回值null
老实说百度的人没回答过我任何专业 问题我这里就悬赏了，看看有没有人回答吧

老实说我是写MSSQL的，语法有不同。
所以，我想问一下
DECLARE OFF INT default 0;
里的OFF和

SELECt c.stat INTO OFF FROM competition c WHERe c.id = cid;

里的OFF是一样的么？
一个是变量一个是临时表吧。。。

declare @data varchar(20) set @data = '' select case when @data is null then null when @data = '' then 'data is empty' end 或者: case when (id is null) then 'yes' else 'no' end as id_value,