mysql - 无法从触发器内的 SELECT 查询获取值

Question

我正在编写一个触发器，以在插入另一个表后更新表中的行。以下是表格的脚本:

表格inv_cost

CREATE TABLE inv_cost (
  Username varchar(20) NOT NULL DEFAULT '',
  MachineType varchar(2) NOT NULL,
  Cost smallint(4) NOT NULL DEFAULT '0',
  PRIMARY KEY (Username,MachineType),
)

表格调查

CREATE TABLE investigation (
  Username varchar(20) NOT NULL,
  MachineType varchar(2) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
DELIMITER ;;
CREATE TRIGGER TRG1 AFTER INSERT ON investigation FOR EACH ROW BEGIN
    DECLARE cost INT DEFAULT 0;
    SET cost = (SELECT Cost FROM inv_cost WHERE Username = NEW.Username AND MachineType = NEW.MachineType);
    UPDATE test SET Balance = Balance - cost WHERE Username = New.Username;
END;;
DELIMITER ;

表调查员

CREATE TABLE test (
  ID int(10) unsigned NOT NULL AUTO_INCREMENT,
  Username varchar(20) NOT NULL DEFAULT '',
  Balance smallint(6) DEFAULT NULL,
  PRIMARY KEY (ID)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;
DELIMITER ;;
CREATE TRIGGER TRG2 AFTER UPDATE ON test FOR EACH ROW BEGIN
    IF NEW.Balance > OLD.Balance THEN
        INSERT INTO payments SET Username = NEW.Username, PaymentOn = NOW(), Amount = NEW.Balance - OLD.Balance;
    END IF;
END;;
DELIMITER ;

触发器 TRG1 的问题在于，它不会根据 SELECT 语句计算变量 cost 的值，并且始终采用值 0 或设置为的任何值其声明中的 DEFAULT。下一行的 UPDATE 查询运行良好(无论 cost 从其声明中获取什么值，或者如果为它分配了一个 const 值，例如 SET cost = 100;)。单独运行的 SELECT 给出所需的 Cost 值。

这段代码有什么问题？

提前致谢。

Answer 1

您必须使用语法SELECT 字段 FROM 表 WHERE 条件 INTO 变量

DROP TRIGGER TRG1 IF EXISTS;
DELIMITER $$
 CREATE TRIGGER TRG1
 AFTER INSERT ON investigation
 FOR EACH ROW BEGIN
      DECLARE cst smallint;
      SELECT Cost FROM inv_cost WHERE Username = NEW.Username AND MachineType = NEW.MachineType INTO cst;
      UPDATE test SET Balance = Balance - cst WHERE Username = New.Username;
 END$$
 DELIMITER ;