MySQL详细讲解变量variables的用法

show variables [like 'pattern'];

mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | ON    |
+---------------+-------+

select @@变量名;

select @@autocommit;
+--------------+
| @@autocommit |
+--------------+
|            1 |
+--------------+

-- 只针对当前客户端当次连接有效
set 变量名 = 值;
-- eg:
set autocommit = 'off';
show variables like 'autocommit';

-- 所有客户端，都有效
set global 变量名 = 值;
set @@global.变量名 = 值;
-- eg:
set global autocommit = 'off';

-- 定义用户变量
set @变量名 = 值;
set @age = 23;

set @变量名 := 值;
set @name := 'Tom';

-- 1、赋值且查看赋值过程
select @变量1 := 字段1, @变量2 := 字段2 from 表 where 条件
select @name := name, @age := age from my_student limit 1;
+---------------+-------------+
| @name := name | @age := age |
+---------------+-------------+
| 刘备          |          18 |
+---------------+-------------+
--- 2、只赋值不看过程
select 字段1, 字段2 from 表 where 条件 into @变量1, @变量2;
select name, age from my_student limit 1 into @name, @age;

select @变量名
mysql> select @name, @age;
+--------+------+
| @name  | @age |
+--------+------+
| 刘备   |   18 |
+--------+------+

declare 变量名 数据类型 [属性];

set @name = '张三';
create function get_name() returns char(4)
return @name;
select get_name();
+------------+
| get_name() |
+------------+
| 张三       |
+------------+

use mydatabase2;
mysql> select @name;
+--------+
| @name  |
+--------+
| 张三   |
+--------+ 

set global 变量名 = 值；
set @@global.变量名 = 值;