mysql> <code><strong>SET @t1=1, @t2=2, @t3</strong>:=<strong>4;</strong></code>mysql> <code>SELECT @t1, @t2, @t3, @t4 := @t1+@t2+@t3;</code>+------+------+------+--------------------+| @t1  | @t2  | @t3  | @t4 := @t1+@t2+@t3 |+------+------+------+--------------------+|    1 |    2 |    4 |                  7 | +------+------+------+--------------------+

SET @pos <strong>:= </strong>0;  #这里用等号，感觉像是逻辑判断，结果也不对了<br>SELECT @pos:=@pos+1 as rank,name FROM players ORDER BY score DESC;

<strong></strong>

SELECT @total := COUNT(*)     FROM  table_name;  # simicolon 分割连个语句SELECT     table_name.id    COUNT(*) AS 'count',    COUNT(*) /     <strong>(SELECT         @total)</strong> AS percent FROM    table_name,WHERE 1=1GROUP BY YEAR(birthday)ORDER BY YEAR(birthday)

注意上面这个SQL语句看起来逻辑清晰，但与下面的语句执行效果和时间都一样（可能MySQL内部优化了）

SELECT     table_name.id    COUNT(*) AS 'count',    COUNT(*) /     <strong>(SELECT </strong>

        COUNT(*) 

     FROM 

        table_name) AS percent 

FROM    table_name,WHERE 1=1GROUP BY YEAR(birthday)ORDER BY YEAR(birthday)

其他一些例子：

http://www.mysqldiary.com/user-defined-variables/

疑问：

这里的变量只能保存一个结果值，如何才能临时保存一个select出的结果集呢。

当然简单的方法是创建表/视图； 或者临时表 ，还有好的方法呢？ 待研究。

DECLARE @var_name  var_type

这里举个例子：

<strong>例1：</strong>

DECLARE @total INT DECLARE @total_distinct INTSELECT     @total:=COUNT(lice_no)    #using ":=" notationFROM    table_name;    SELECT @total_distinct:=COUNT( DISTINCT lice_no)   #using ":=" noationsFROM table_name; SELECT @total - @total_distinct

<strong>例2：</strong>

DECLARE @register_count INT;DECLARE @total_count INT;SELECT     @register_count := COUNT(1) FROM    t1 WHERE id > 10 ;

    SELECT     @total_count := COUNT(1) FROM    t1 ;    SELECT     (@register_count * @total_count) AS ratio2

<ol>
<li>DECLARE 必须指定类型，而SET是不用的</li>
<li>SET定义的是用户自定义变量，是Session Sensitive 的； DECLARE 声明的变量一般为局部变量，其有效区间是声明的函数或存储过程中。</li>
<li>定义全局变量应该为 SET GLOBAL @var_name 后者SET @@GLOBAL.var_name</li>
</ol>