MySQL 是否允许在 C 中进行回调，以便在发生更改时可以通知我?

素材狗 2022-01-01 08:34:29 文章分类：数据库问题点击数：3次

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Does MySQL permit callbacks in C such that when a change happens, I can be notified?(MySQL 是否允许在 C 中进行回调，以便在发生更改时可以通知我?)

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问题描述

MySQL 是否允许 C 中的回调，以便当数据库中发生更改(例如插入)由不同程序或用户在命令行执行时，我可以收到通知?

Does MySQL permit callbacks in C such that when a change happens in the database, like an insert, that is performed by a different program or by the user at the command line, I can be notified?

我猜不是，因为 mysqlclient 是一个库，而不是一个正在运行的线程.但我不妨问问.

I am guessing that it doesn't, because mysqlclient is a library, not a running thread. But I may as well ask.

推荐答案

像这样创建触发器.

DELIMITER $$

CREATE TRIGGER ad_mytable_each AFTER DELETE ON MyTable FOR EACH ROW
BEGIN
  #write code that trigger After delete (hence the "ad_" prefix)
  #For table MyTable (The _MyTable_ middle)
  #On each row that gets inserted (_each suffix)
  #
  #You can see the old delete values by accesing the "old" virtual table.
  INSERT INTO log VALUES (old.id, 'MyTable', old.field1, old.field2, now());

END$$

DELIMITER ;

有INSERT、DELETE、UPDATE
的触发器他们可以触发 BEFORE 或 AFTER 操作.
触发器BEFORE动作可以通过强制错误来取消动作，就像这样.

There are triggers for INSERT, DELETE, UPDATE
And they can fire BEFORE or AFTER the action.
The trigger BEFORE the action can cancel the action by forcing an error, like so.

CREATE TRIGGER bd_mytable_each BEFORE DELETE ON MyTable FOR EACH ROW
BEGIN
  #write code that trigger Before delete (hence the "db_" prefix)
  declare DoError Boolean; 

  SET DoError = 0;

  IF old.id = 1 THEN SET DoError = 1; END IF; 

  IF (DoError = 1) THEN SELECT * FROM Table_that_does_not_exist_to_force_error;
  #seriously this example is in the manual.

END$$

DELIMITER ;

这将防止删除记录 1.

This will prevent deletion of record 1.

更新前触发器甚至可以更改更新的值.

A before UPDATE Trigger can even change the values updated.

CREATE TRIGGER bu_mytable_each BEFORE UPDATE ON MyTable FOR EACH ROW
BEGIN
  IF new.text = 'Doon sucks' THEN SET new.text = 'Doon rules';
END$$

DELIMITER ;