MySQL learning notes (1) -- variables

mysql Learning notes (1)—— Variable

One 、 Classification of variables :

stay mysql The variables in are divided into System variables and Custom variable .
The system variables are divided into Global variables and Session variables
Custom variables are divided into User variables and local variable

Two 、 System variables

Global variables

（1） Concept and classification ：

System variables ： Provided by the system , Data server level .
The system variables are divided into Global variables and Session variables

（2） Commonly used sql sentence ：

1. Look at all the system variables
show global [session] variables;

2. Look at some of the system variables that meet the criteria （ Fuzzy query ）
show global [session] variables like ‘%char%’

3. View the value of a specified system variable
select @@global [session]. System variable name

4. Assign a value to a system variable
Mode one ：
set global [session] System variable name = value ;

Mode two ：
set @@global [session]. System variable name = value ;

（3） demonstration ：

--  View global variables 
SHOW GLOBAL VARIABLES;

--  Look at some of the global variables 
SHOW GLOBAL VARIABLES LIKE '%char%';

--  View the value of the specified global variable 
SELECT @@global.autocommit;
SELECT @@tx_isolation; --  View the isolation level of the transaction 

--  Assign a value to a specified global variable 
SET @@global.autocommit = 0; --  Turn off automatic transaction commit 

SELECT @@global.autocommit; -- 0

（4） Scope

Every time the server starts, it will assign initial values to all global variables , For all conversations （ Connect ） It works , But not across servers .

For example, I changed the automatic submission of Alibaba cloud server sessions , Set up in order to 0, But in the local session, the value of automatic submission is 1

Session variables

demonstration

--  Session variables 
SHOW SESSION VARIABLES;
SHOW VARIABLES;

You can see that there are more than ten global variables in the system

--  Look at some session variables 
SHOW SESSION VARIABLES LIKE '%char%';

--  View the specified session variable 
SELECT @@session.tx_isolation;
SELECT @@tx_isolation;

--  Assign a value to a session variable 
--  Mode one ：
SET @@session.tx_isolation='read-uncommitted';
--  Mode two ：
SET SESSION tx_isolation='read-committed';

--  Change back ( Repeatable )
SET SESSION tx_isolation='repeatable-read';

3、 ... and 、 Custom variable

Concept ：

Custom variable ：
explain ： Variables are user-defined
Use ： Declare variables assignment Use （ see 、 Compare 、 Operations, etc ）

User variables

(1) Scope

User variables ：
Scope ： For the current session （ Connect ） It works , Same as the scope of the conversation variable

(2) Use steps

Declare and initialize ：

Assignment operator ： = or :=( Recommended )

set @ User variable name = value ; or
set @ User variable name := value , or
select @ User variable name := value

assignment ：
Mode one ： adopt set or select
set @ User variable name = value ; or
set @ User variable name := value , or
select @ User variable name := value

Mode two ： adopt select into
select Field into @ User variable name from surface ;

3. Look at the value of the user variable
select @ User variable name ;

(3) demonstration

--  Case study 
--  Mode one 
SET @name='jack';

SELECT @name;

--  Mode two 
SELECT @count:=0;
SELECT COUNT(*) INTO @count FROM stu;
SELECT @count;

local variable

(1) Scope

Just defining it begin end Effective in ( First sentence )

(2) Use

– Statement ：
declare Variable name type ;
declare Variable name type default value ;

– assignment ( Similar to user variables )
Mode one ： adopt set or select
set Local variable name = value ; or
set Local variable name := value , or
select @ Local variable name := value

Mode two ： adopt select into
select Field into Local variable name from surface ;

see
select Local variable name ;

(2) demonstration

--  User variables 
SET @a=1;
SET @b=2;
SET @sum= @a + @b;
SELECT @sum;

Compare user variables with local variables

Column 1	Scope	Where to define and use	grammar
User variables	Current session	Anywhere in the conversation	Have to add ’@' Symbol . There is no need to qualify the type
local variable	begin end in	Only in begin end in , And for the first sentence	Generally, there is no need to add ’@' Symbol , You need to qualify the type

Test with local variables ：

-- 2. local variable ( The following operations are not supported , Because the scope of the local variable is begin end in )
DECLARE a INT DEFAULT 1;
DECLARE b INT DEFAULT 2;
DECLARE SUM INT;
SET SUM = a + b;
SELECT SUM;

Wrong report , It shows that the scope of local variables is limited