MySQL存储过程中变量的定义以及应用详解

SHOW [ SESSION | GLOBAL ] VARIABLES 

SHOW [ SESSION | GLOBAL ] VARIABLES LIKE '变量名'

SELECT @@[SESSION | GLOBAL].变量名

SET [ SESSION | GLOBAL ] 系统变量名 = 值 
SET @@[SESSION | GLOBAL].系统变量名 = 值 

mysql> show variables;

mysql> show variables like '%auto%';
+----------------------------------------------+-------+
| Variable_name                                | Value |
+----------------------------------------------+-------+
| auto_generate_certs                          | ON    |
| auto_increment_increment                     | 1     |
| auto_increment_offset                        | 1     |
| autocommit                                   | ON    |
| automatic_sp_privileges                      | ON    |
| caching_sha2_password_auto_generate_rsa_keys | ON    |
| innodb_autoextend_increment                  | 64    |
| innodb_autoinc_lock_mode                     | 2     |
| innodb_stats_auto_recalc                     | ON    |
| sha256_password_auto_generate_rsa_keys       | ON    |
| sql_auto_is_null                             | OFF   |
+----------------------------------------------+-------+
11 rows in set (0.00 sec)

mysql> select @@session.autocommit;
+----------------------+
| @@session.autocommit |
+----------------------+
|                    1 |
+----------------------+
1 row in set (0.00 sec)

mysql> set @@session.autocommit = 0;
Query OK, 0 rows affected (0.02 sec)
或者
mysql> set session autocommit = 0;
Query OK, 0 rows affected (0.00 sec)

SET @变量名1 = 变量值1
SET @变量名1 = 变量值1 , @变量名2 = 变量值2

SET @变量名1 := 变量值1 
SET @变量名1 := 变量值1 , @变量名2 := 变量值2 

建议使用:=的方式为变量赋值，因为在变量运算时=符号也是运算符号。

SELECT @变量名1 := 变量值1 , @变量名2 := 变量值2 

SELECT 字段名 INTO @变量名 FROM 表名
将某个表的某个字段的结果赋值给变量。

SELECT @变量名1

1.一次性定义一个变量
mysql> set @name = 'jiangxl';
或者
mysql> set @username :=  'wang';
2.一次性定义多个变量
mysql> set @username = 'wang' , @userid = '1';
或者
mysql> set @age := '19' , @book := 'linux';

1.一次性定义一个变量
mysql> select @myzhiwu := 'devops_linux';
+----------------------------+
| @myzhiwu := 'devops_linux' |
+----------------------------+
| devops_linux               |
+----------------------------+

2.一次性定义多个变量
mysql> select @myzhiwu := 'devops_linux' , @myage := '25';
+----------------------------+----------------+
| @myzhiwu := 'devops_linux' | @myage := '25' |
+----------------------------+----------------+
| devops_linux               | 25             |
+----------------------------+----------------+

mysql> select count(*) into @mycount from ryxxb;

mysql> select @name,@username,@userid,@age,@book,@myzhiwu,@myage;
+---------+-----------+---------+------+-------+--------------+--------+
| @name   | @username | @userid | @age | @book | @myzhiwu     | @myage |
+---------+-----------+---------+------+-------+--------------+--------+
| jiangxl | wang      | 1       | 19   | linux | devops_linux | 25     |
+---------+-----------+---------+------+-------+--------------+--------+

DECLARE 变量名 变量值的类型 [DEFAULT 默认值]
变量值的类型就是数据库字段的类型，例如：INT BIGINT CHAR VARCHAR DATE TIME等等。

SET 变量名 = 变量值
SET 变量名 := 变量值
SELECT 字段名 INTO 变量名 FROM 表名;

1.创建视图
create procedure proc_2()
begin
	declare ryxxb_count int default 0;			#声明ryxxb表总数的变量，类型为int，默认值为0
	declare username varchar(10);				#声明username变量，类型为varchar
	set username := 'jiangxl';						#赋值username变量
	select count(*) into ryxxb_count from ryxxb;		#将ryxxb表的总数据量赋值为ryxxb_count变量
	select ryxxb_count,username;						#查询变量的值
end;

2.调用视图
mysql> call proc_2();
+-------------+----------+
| ryxxb_count | username |
+-------------+----------+
|          22 | jiangxl  |
+-------------+----------+
1 row in set (0.00 sec)

DECLARE total_count INT DEFAULT 0
 SET total_count = 10;

DECLARE total_products INT DEFAULT 0
 SELECT COUNT(*) INTO total_products
 FROM products