Cannot fetch a value from a SELECT query inside a trigger

问题

I am writing a trigger to UPDATE a row in a table AFTER INSERT on another table. Here are the scripts of the tables:

Table 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),
)

Table investigation

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 ;

Table investigator

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 ;

The problem with trigger TRG1 is that it does not compute the value of the variable cost from the SELECT statement, and always takes the value 0 or whatever is set to DEFAULT in its declaration. The UPDATE query on the next line works good (with whatever value cost takes from its declaration OR if it is assigned a const value like SET cost = 100;). The SELECT run separately gives the required value of Cost.

What is wrong with this code?

Thanks in advance.

回答1:

You have to use the sintax SELECT field FROM table WHERE condition INTO variable

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 ;

来源：https://stackoverflow.com/questions/9977690/cannot-fetch-a-value-from-a-select-query-inside-a-trigger

标签

mysql

triggers