SQL-變量
變量
1.系統變量:
全局變量:作用域:服務器每次啓動將爲所有的全局變量賦初始值,
針對於所有的的會話(連接)有效,但不垮重啓。
會話變量:僅僅針對當前的會話(連接)有效
2.自定義變量:
用戶變量
局部變量
一. 系統變量
即該變量由系統提供,不是用戶定義,屬於服務層面
1. 使用語法:
1.查看所有的系統的變量
show grobal|[session] variables;
注意:如果是全局級別,則需要要加global,如果是會話級別,則需要加session,如果不寫,則默認session。
2.查看滿足條件的部分系統變量
show global | [session] variables like '要查看變量的部分字符' ;
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.查看指定的某個系統變量的值
select @@global | [session] .系統變量名
mysql> select @@global.character_set_system;
+-------------------------------+
| @@global.character_set_system |
+-------------------------------+
| utf8 |
+-------------------------------+
1 row in set (0.00 sec)
4.爲某個系統變量賦值
方式一:
set global|[session] 系統變量名=值;
方式二:
set @@global|[session].系統變量名=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. 自定義變量
(1)用戶變量
1.即用戶自己定義的,不是用系統提供的。
2.使用步驟:
聲明
賦值
使用(查看,比較,運算等)
3.作用域:
針對當前會話(連接)有效,同於會話變量的作用域
賦值操作符:= 或 :=
(1)聲明並初始化
set @用戶變量名=值; 或
set @用戶變量名 := 值;或
select @用戶變量名 := 值;
(2)賦值(更新用戶變量的值)
方式一(聲明並初始化一樣):通過set或select
set @用戶變量名=值;或
set @用戶變量名 := 值;或
select @用戶變量名 := 值;
方式二:通過select into
select 字段 into 變量名 from 表;
(3)使用:select @用戶變量名;
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> #賦值
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.局部變量
作用域:僅僅在定義它的 begin end 中有效
應用在begin end 中的第一句話
(1)聲明:
declare 變量名 類型;
declare 變量名 類型 default 值;
(2)賦值
方式一:
通過set或select
set 局部變量名=值;或
set 局部變量名 := 值;或
select @局部變量名 := 值;
方式二:通過select into
select 字段 into 局部變量名 from 表;
(3)使用
select 局部變量名;
練習:聲明兩個變量並賦初始值,求和,並打印;
@用戶變量名
set @n=1;
set @m=2;
set @sum =@n+@m;
select @sum;
#2.局部變量
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. 因爲它得在begin end 裏