mysql--variable

First, System variables
Description: Variables are defined by the system, not user defined, and belong to the server level
Note: Global variables need to be added to the GLOBALS keyword, session variables need to be added to the sessions keyword, if not written, the default session level
Steps to use:
1. View all system Variables
Show global| "Session" variables;
2. View some system variables that meet the criteria
Show global| "session" variables like '%char% ';
3. View the value of the specified system variable
SELECT @ @global | "Session" system variable name;
4. Assigning a value to a system variable
Way One:
Set global| "session" System variable name = value;
Way two:
SET @ @global | "Session" SYSTEM variable name = value;
1.1 Global variables
Scope: Valid for all sessions (connections), but not across reboots
Precise use of select imprecise usage show
① View All global variables
SHOW GLOBAL VARIABLES;
② viewing partial system variables that meet the criteria
SHOW GLOBAL VARIABLES like '%char% ';
③ View the value of a specified system variable
SELECT @ @global. autocommit;
④ Assigning a value to a system variable
SET @ @global. autocommit=0;
SET GLOBAL autocommit=0;
1.2 Session Variables
Scope: Valid for current session (connection)
① View all Session variables
SHOW SESSION VARIABLES;
② viewing partial session variables that meet the criteria
SHOW SESSION VARIABLES like '%char% ';
③ View the value of the specified session variable
SELECT @ @autocommit;
SELECT @ @session. tx_isolation;
④ Assigning a value to a session variable
SET @ @session. tx_isolation= ' read-uncommitted ';
SET SESSION tx_isolation= ' read-committed ';
Ii.. Custom variables
Description: The variable is customized by the user, not the system-supplied
Steps to use:
1. Statement
2. Assign Value
3. Use (view, compare, calculate, etc.)
2.1 User Variables
Scope: Valid for current session (connection), scoped to session variable
Assignment operator: = or: =
① declaring and initializing
SET @ variable name = value;
SET @ variable Name: = value;
SELECT @ variable Name: = value;
② Assignment (update the value of the variable)
Way One:
SET @ variable name = value;
SET @ variable Name: = value;
SELECT @ variable Name: = value;
Way two:
SELECT field into @ variable name
from table;
③ (View the value of a variable)
SELECT @ variable name;
2.2 Local Variables
Scope: Valid only in the begin end block that defines it
Apply the first sentence in the Begin end
② Assignment (update the value of the variable)
Way One:
SET local variable name = value;
SET local variable name: = value;
SELECT local variable name: = value;
Way two:
SELECT field into with variable name
from table;
③ (View the value of a variable)
SELECT local variable name;

Comparison of user variables and local variables

mysql--variable