Operation of variables in MySQL

Variables are divided into system variables and user-defined variables , seeing the name of a thing one thinks of its function , System variables are variables defined by the system , These variables are fixed , User variables are not fixed , Users can define and use .

• System variables

System variables are divided into global system variables and session system variables , The modification of global system variables affects all sessions , The modification of session variables only affects the current session .

1. Look at the system variables

show global variables

2. Filter on view

show global variables like '%char%'

3. View session variables

show session variables 

Be careful ： If global or session Omit , The default is session variable

4. View a specific variable

select @@global.max_connections;

System variables are connected by @ start ,@@ And then global To view system variables ,@@ And then session Is to view the session variables .

5. Change the value of the variable

set @@global .max_connections = 3000;

Look again

• User variables

• User session variables , After user declaration , As long as the current connection is not closed , It will always be stored in the server memory , The session is isolated from the variables before the session .

1. User session variable definition

set @count = 0;
set @count := 1;
select  @count;

2. Define user variables , Batch update sql

set @indexcount := 0;

update emp set count = (@indexcount := @indexcount + 1);

select *
from emp;

• User defined variables can only be used in begin end in Not yet .