觸發器：引用更新的屬性

Question

我在UPDATE上有一個觸發器。

從UPDATE SQL命令未更新的表中引用屬性的正確過程是什麼？該屬性是否仍然在UPDATE變量中？我想爲更新的行獲取該屬性的值。

Answer 1

通過使用關鍵字OLD和NEW，可以在更新之前和更新MySQL之後訪問列的值。

例如，如果你想確定是否列的值實際上已經改變時更新，你可以做

IF NOT OLD.column_name <=> NEW.column_name THEN 
    -- do something here 
END IF; 

注：<=>是NULL安全的比較操作在MySQL

BTW： MySQL中沒有UPDATED虛擬表。它來自SQL Server。

這裏是一個SQLFiddle演示。 請注意，儘管更新會影響表中的所有記錄，但log表中只記錄了一條消息。這是因爲最終id爲2的行的價值保持不變。

---

UPDATE：，讓您的finished標誌同步，你需要對所有的事件（插入，更新，刪除）的觸發器。

DELIMITER // 
CREATE TRIGGER tg_ai_event 
AFTER INSERT ON event 
FOR EACH ROW 
BEGIN 
    UPDATE activity a 
    SET status = (EXISTS(SELECT * 
          FROM event 
          WHERE activity = a.activity_id 
          AND done = 0)) 
    WHERE activity_id = NEW.activity; 
END// 

CREATE TRIGGER tg_ad_event 
AFTER DELETE ON event 
FOR EACH ROW 
BEGIN 
    UPDATE activity a 
    SET status = (EXISTS(SELECT * 
          FROM event 
          WHERE activity = a.activity_id 
          AND done = 0)) 
    WHERE activity_id = OLD.activity; 
END// 

CREATE TRIGGER tg_au_event 
AFTER UPDATE ON event 
FOR EACH ROW 
BEGIN 
    IF NOT OLD.activity <=> NEW.activity THEN 
    -- if activity id was changed for an event then clculate finished flag 
    -- for both old and new activity id 
    UPDATE activity a 
     SET status = (EXISTS(SELECT * 
           FROM event 
          WHERE activity = a.activity_id 
           AND done = 0)) 
    WHERE activity_id IN(OLD.activity, NEW.activity); 
    ELSE 
    -- otherwise calculate finished flag only if done flag is changed 
    IF NOT OLD.done <=> NEW.done THEN 
     UPDATE activity a 
     SET status = (EXISTS(SELECT * 
           FROM event 
           WHERE activity = a.activity_id 
           AND done = 0)) 
     WHERE activity_id = NEW.activity; 
    END IF; 
    END IF; 
END// 
DELIMITER ; 

這裏是SQLFiddle演示