無法從觸發器內的SELECT查詢中獲取值

Question

我在寫一個觸發器來更新另一個表上的AFTER INSERT表中的行。下面是表的腳本：

表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), 
) 

表調查

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 ; 

表調查

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 ; 

觸發器TRG1的問題在於，它不會從SELECT語句計算變量成本的值，並且始終將值0或任何在其聲明中設置爲DEFAULT的值。下一行的UPDATE查詢工作正常（無論值的代價從其聲明中獲取，或者分配了像SET cost = 100這樣的常量值）。 SELECT運行單獨給出所需的值成本。

這段代碼有什麼問題？

在此先感謝。

Answer 1

您必須使用sintax SELECT場FROM表WHERE條件INTO變量

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 ;