mysql Definition and assignment of variables

have access to declare Keyword to define variables , After definition, you can assign values to variables .

Defining variables

declare my_sql int declare 10;

Assign values to variables

set my_sql =30;
select …into Statement to assign a value to a variable
select id into my_sql from tb_student where id=2;

Variable classification

local variable ：
mysql local variable , Can only be used in begin/end In the block .
User variables ：
mysql User variables in do not need to be declared in advance , Use... Directly when using “@ Variable name ” Just use it .

 Such as ：@test=1;

Session variables ：
mysql Session variables , The server maintains a series of session variables for each connected client , Its scope is limited to the current connection , That is, the session variables in each connection are independent .

• Show session variables ：

show session variables

• Query session variables ：

select @@auto_increment_increment;
select@@session.auto_increment_increment;
show session variables like '%auto_increment_increment%'; -- session Keywords can be omitted 

• Set session variables ：

set session auto_increment_increment=1;
set @@session.auto_increment_increment=2;
set auto_increment_increment=3; --  When omitting session When a keyword , The default is session, That is, set the value of the session variable 

--  keyword session You can also use keywords local Instead of 
set @@local.auto_increment_increment=1;
select @@local.auto_increment_increment;

Global variables ：
Global variables affect the overall operation of the server , When the service starts , It initializes all global variables to default values . To change global variables , Must possess super jurisdiction .

• Show global variables ：

show global variables;

• Set global variables ：

set global sql_warnings=ON; -- global Don't omit 
set@@global.sql_warnings=OFF;

• Query global variables ：

--  There are two ways to query the value of a global variable 
select @@global.sql_warnings;
show global variables like '%sql_warnings%';