Mysql variable introduction and use of

variable

System Variables

Variables provided by the system, not the user-defined, belong to the server level

• Global Variables

  • View all global variables

    • SHOW GLOBAL VARIABLES;
      

  • Scope: Each time you start the server for all global variables will be assigned an initial value (connection) valid for all sessions, but can not cross restart

• Session Variables

  • View all session variables

    • SHOW SESSION VARIABLES;
      

• Check the condition of the system variables

  • SHOW GLOBAL|SESSION VARIABLES LIKE '%关键字%'
    

• Check the specified value of a system variable

  • SELECT @@global|session.系统变量名;
    

• Assigned to system variables

  • SET global|session 系统变量名=值;
    

  • SET @@global|session.系统变量名=值;
    

• note

  • Global level requires home global, session level need to add session, do not write the default session

Custom Variables

• Description: The user-defined variables are not assigned by the system

• User Variables

  • Scope: Scope for the current session (connection) is valid, in the same session variables

  • Declare and initialize

    • SET 用户变量名=值;
      SET 用户变量名:=值;
      SELECT @用户变量名:=值;
      

  • Assignment

    • By SET or SELECT, ibid.

    • SELECT 字段 INTO 变量名
      FROM 表;
      

  • View

    • SELECT @变量名;
      

  • Steps for usage:

    • statement
    • Assignment
    • Use (view, compare, arithmetic)

• Local variables

  • Scope: it is defined only in the effective begin end

  • Applications begin end of the first sentence

  • statement:

    • DECLARE 变量名 类型;
      DECLARE 变量名 类型 DEFAULT 值;
      

  • Assignment

    • @ symbol set manner may be omitted
    • can not be omitted select

  • use

    • SELECT 局部变量名;