mysql - 触发: Referencing updated attribute

Question

我有一个更新触发器。

从 UPDATE SQL 命令未更新的表中引用属性的正确过程是什么？该属性是否仍在 UPDATE 变量中？我想获取更新行的该属性的值。

Answer 1

您可以使用关键字OLD来访问MySQL中更新前和更新后的列的值和NEW .

例如，如果您想确定列的值在更新期间是否实际上已更改，您可以这样做

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

注意: <=> MySQL 中的 NULL 安全比较运算符

顺便说一句:没有 UPDATED MySQL 中的虚拟表。它来自 SQL Server。

这是一个 SQLFiddle演示。 请注意，即使更新影响了表中的所有记录，log 中仅记录了一条消息 table 。这是因为 id 为 2 的行的值最终保持不变。

<小时/>

更新:保留您的 finished同步标记您需要所有事件(插入、更新、删除)的触发器。

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 ;

这里是SQLFiddle 演示