SQL-变量(16)

变量

1.系统变量:
        全局变量:作用域:服务器每次启动将为所有的全局变量赋初始值,
               针对于所有的的会话(连接)有效,但不垮重启。
        
        会话变量:仅仅针对当前的会话(连接)有效
2.自定义变量:
        用户变量
        局部变量

一. 系统变量

即该变量由系统提供,不是用户定义,属于服务层面            

1. 使用语法:

1.查看所有的系统的变量
show grobal|[session] variables;
注意:如果是全局级别,则需要要加global,如果是会话级别,则需要加session,如果不写,则默认session。
mysql> show global variables;
+----------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Variable_name                                            | Value                                                                                                                                                                                                                                                                                                                                                                                                                                      |
+----------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| activate_all_roles_on_login                              | OFF                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| auto_generate_certs                                      | ON                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| auto_increment_increment                                 | 1                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| auto_increment_offset                                    | 1                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| autocommit                                               | ON 
...
...
531 rows in set (0.01 sec)

mysql> show session variables;
+----------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Variable_name                                            | Value                                                                                                                                                                                                                                                                                                                                                                                                                                      |
+----------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| activate_all_roles_on_login                              | OFF                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| auto_generate_certs                                      | ON                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| auto_increment_increment                                 | 1                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| auto_increment_offset                                    | 1                                                                                                                                                                                                                                                                                                                                                             
...
...
548 rows in set (0.00 sec)                                                                                                                                                                 
2.查看满足条件的部分系统变量
show global | [session] variables like '要查看变量的部分字符' ;
mysql> show global variables like '%char%';
+--------------------------+-----------------------------------------------------------+
| Variable_name            | Value                                                     |
+--------------------------+-----------------------------------------------------------+
| character_set_client     | utf8mb4                                                   |
| character_set_connection | utf8mb4                                                   |
| character_set_database   | utf8mb4                                                   |
| character_set_filesystem | binary                                                    |
| character_set_results    | utf8mb4                                                   |
| character_set_server     | utf8mb4                                                   |
| character_set_system     | utf8                                                      |
| character_sets_dir       | /usr/local/mysql-8.0.11-macos10.13-x86_64/share/charsets/ |
+--------------------------+-----------------------------------------------------------+
8 rows in set (0.01 sec)


3.查看指定的某个系统变量的值
select @@global | [session] .系统变量名

mysql> select @@global.character_set_system;
+-------------------------------+
| @@global.character_set_system |
+-------------------------------+
| utf8                          |
+-------------------------------+
1 row in set (0.00 sec)
4.为某个系统变量赋值
  方式一:
  set global|[session] 系统变量名=值;
  方式二:
  set @@global|[session].系统变量名=值;                     
mysql> show global variables like '%isolation%';
+-----------------------+-----------------+
| Variable_name         | Value           |
+-----------------------+-----------------+
| transaction_isolation | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.00 sec)
mysql> set global transaction_isolation ='read-committed';
Query OK, 0 rows affected (0.00 sec)

mysql> show global variables like '%isolation%';
+-----------------------+----------------+
| Variable_name         | Value          |
+-----------------------+----------------+
| transaction_isolation | READ-COMMITTED |
+-----------------------+----------------+
1 row in set (0.00 sec)


mysql> set @@global.transaction_isolation='REPEATABLE-READ';
Query OK, 0 rows affected (0.07 sec)

mysql> show global variables like '%isolation%';
+-----------------------+-----------------+
| Variable_name         | Value           |
+-----------------------+-----------------+
| transaction_isolation | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.00 sec)


二. 自定义变量

(1)用户变量

1.即用户自己定义的,不是用系统提供的。
2.使用步骤:
  声明
  赋值
  使用(查看,比较,运算等)
3.作用域:
  针对当前会话(连接)有效,同于会话变量的作用域 
赋值操作符:= 或 := 

(1)声明并初始化
 set @用户变量名=值; 或
 set @用户变量名 := 值;或
 select @用户变量名 := 值;
(2)赋值(更新用户变量的值)
 方式一(声明并初始化一样):通过set或select
       set @用户变量名=值;或
       set @用户变量名 := 值;或
       select @用户变量名 := 值;
方式二:通过select into 
      select 字段 into 变量名 from 表;
(3)使用:select  @用户变量名;           
  

案例

mysql> set @name ='keen';
Query OK, 0 rows affected (0.06 sec)

mysql> set @name =22;
Query OK, 0 rows affected (0.00 sec)

mysql> set @count =1;
Query OK, 0 rows affected (0.00 sec)

mysql> #赋值
mysql> use myemployees;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> select count(*) into @count from employees;
Query OK, 1 row affected (0.06 sec)

mysql> #查看
mysql> select @count;
+--------+
| @count |
+--------+
|    107 |
+--------+
1 row in set (0.00 sec)

mysql> select @name;
+-------+
| @name |
+-------+
|    22 |
+-------+
1 row in set (0.00 sec)

mysql> set @name='keen';
Query OK, 0 rows affected (0.00 sec)

mysql> select @name;
+-------+
| @name |
+-------+
| keen  |
+-------+
1 row in set (0.00 sec)


2.局部变量

 作用域:仅仅在定义它的 begin end 中有效
 应用在begin end 中的第一句话

 (1)声明:
    declare 变量名 类型;
    declare 变量名 类型 default 值;
(2)赋值
   方式一:
    通过set或select
    set 局部变量名=值;或
    set 局部变量名 := 值;或
    select @局部变量名 := 值;
   方式二:通过select into 
    select 字段 into 局部变量名 from 表;    
(3)使用
  select 局部变量名;

练习:声明两个变量并赋初始值,求和,并打印;

@用户变量名
set @n=1;
set @m=2;
set @sum =@n+@m;
select @sum;


#2.局部变量
declare m int default 1;
declare n  int default 2;
declare sum int;
set sum=m+n;
select sum;
//declare is not valid at this position. 因为它得在begin end 里
      

注:这是本人的学习笔记及练习,如果有错误的地方望指出一起讨论,谢谢!

    原文作者:小白201808
    原文地址: https://www.jianshu.com/p/f6934dd0ad51
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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