show variables [like 'pattern'];

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

select @@變量名;

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

-- 只針對(duì)當(dāng)前客戶端當(dāng)次連接有效
set 變量名 = 值;
-- eg:
set autocommit = 'off';
show variables like 'autocommit';

-- 所有客戶端，都有效
set global 變量名 = 值;
set @@global.變量名 = 值;
-- eg:
set global autocommit = 'off';

-- 定義用戶變量
set @變量名 = 值;
set @age = 23;

set @變量名 := 值;
set @name := 'Tom';

-- 1、賦值且查看賦值過程
select @變量1 := 字段1, @變量2 := 字段2 from 表 where 條件
select @name := name, @age := age from my_student limit 1;
+---------------+-------------+
| @name := name | @age := age |
+---------------+-------------+
| 劉備          |          18 |
+---------------+-------------+
--- 2、只賦值不看過程
select 字段1, 字段2 from 表 where 條件 into @變量1, @變量2;
select name, age from my_student limit 1 into @name, @age;

select @變量名
mysql> select @name, @age;
+--------+------+
| @name  | @age |
+--------+------+
| 劉備   |   18 |
+--------+------+

declare 變量名 數(shù)據(jù)類型 [屬性];

set @name = '張三';
create function get_name() returns char(4)
return @name;
select get_name();
+------------+
| get_name() |
+------------+
| 張三       |
+------------+

use mydatabase2;
mysql> select @name;
+--------+
| @name  |
+--------+
| 張三   |
+--------+ 

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