mysql中的变量使用方法

SET @myVariable = 'Hello World';
SELECT @myVariable;

SELECT @myVariable := 'Hello World';
SELECT @myVariable;

SHOW VARIABLES LIKE 'max_connections';

SET GLOBAL max_connections = 200;

[mysqld]
max_connections = 200

SELECT @undefinedVariable;

SET @undefinedVariable = 'Value';
SELECT @undefinedVariable;

DELIMITER //
CREATE PROCEDURE testVar()
BEGIN
  SET @myVar = 'Inside Procedure';
END //
DELIMITER ;
CALL testVar();
SELECT @myVar; -- 这里会返回NULL，因为变量只在存储过程中有效

DELIMITER //
CREATE PROCEDURE testVar(OUT myVar VARCHAR(255))
BEGIN
  SET myVar = 'Inside Procedure';
END //
DELIMITER ;
CALL testVar(@myVar);
SELECT @myVar; -- 现在可以正确返回'Inside Procedure'