可以使用时间戳（旧和新）来检查，当没有更改行时，不会更新。 （可能是混乱的原因，因为那个也被称为'on update'，但是没有发生改变时不执行）
一秒钟内的更改将不会执行触发器的那部分，但在某些情况下（例如，当您有一个应用程序拒绝快速更改时）。

例如，而不是

 如果NEW.a<> OLD.a或NEW.b<> OLD.b / *等等，一直到NEW.z<> OLD.z * / 
 THEN 
 INSERT INTO bar（a，b）VALUES（NEW.a，NEW.b）; 
 END IF 
 

您可以使用

 如果NEW.ts<> OLD.ts 
 THEN 
 INSERT INTO bar（a，b）VALUES（NEW.a，NEW.b）; 
 END IF 
 

然后你不必每次更新

（

  create table foo（a INT，b INT，ts TIMESTAMP）; 
创建表格栏（INT，b INT）; 

 INSERT INTO foo（a，b）VALUES（1,1）; 
 INSERT INTO foo（a，b）VALUES（2,2）; 
 INSERT INTO foo（a，b）VALUES（3,3）; 

 DELIMITER /// 

 CREATE TRIGGER ins_sum AFTER更新后的foo 
每个行
 BEGIN 
如果NEW.ts<> ; OLD.ts THEN 
 INSERT INTO bar（a，b）VALUES（NEW.a，NEW.b）; 
 END IF; 
 END; 
 /// 

 DELIMITER; 

 select * from foo; 
 + ------ + ------ + --------------------- + 
 | a | b | ts | 
 + ------ + ------ + --------------------- + 
 | 1 | 1 | 2011-06-14 09:29:46 | 
 | 2 | 2 | 2011-06-14 09:29:46 | 
 | 3 | 3 | 2011-06-14 09:29:46 | 
 + ------ + ------ + --------------------- + 
集合中的3行0.00 sec）

  -  UPDATE无变化
 UPDATE foo SET b = 3 WHERE a = 3; 
查询OK，0行受影响（0.00秒）
行匹配：1更改：0警告：0 

  - 时间戳没有更改
 select * from foo WHERE a = 3; 
 + ------ + ------ + --------------------- + 
 | a | b | ts | 
 + ------ + ------ + --------------------- + 
 | 3 | 3 | 2011-06-14 09:29:46 | 
 + ------ + ------ + --------------------- + 
集合中的1行0.00秒）

  - 触发器没有运行
 select * from bar; 
空集（0.00秒）

  - 更改更新
 UPDATE foo SET b = 4 WHERE a = 3; 
查询OK，1行受影响（0.00秒）
行匹配：1更改：1警告：0 

  - 更改时间戳
 select * from foo; 
 + ------ + ------ + --------------------- + 
 | a | b | ts | 
 + ------ + ------ + --------------------- + 
 | 1 | 1 | 2011-06-14 09:29:46 | 
 | 2 | 2 | 2011-06-14 09:29:46 | 
 | 3 | 4 | 2011-06-14 09:34:59 | 
 + ------ + ------ + --------------------- + 
集合中的3行0.00秒）

  - 并且触发器运行
 select * from bar; 
 + ------ + ------ + --------------------- + 
 | a | b | ts | 
 + ------ + ------ + --------------------- + 
 | 3 | 4 | 2011-06-14 09:34:59 | 
 + ------ + ------ + --------------------- + 
集合中的1行0.00 sec）
 

它是工作，因为mysql的行为处理时间戳。
只有在更新中发生更改时，才会更新时间戳。

文档位于此处：

https://dev.mysql.com/doc/refman/5.7/en/timestamp-initialization .html

  desc foo; 
 + ------- + ----------- + ------ + ----- + ------------ ------- + ----------------------------- + 
 |字段|类型| Null | Key |默认|额外| 
 + ------- + ----------- + ------ + ----- + ------------ ------- + ----------------------------- + 
 | a | int（11）| YES | | NULL | | 
 | b | int（11）| YES | | NULL | | 
 | ts | timestamp | NO | | CURRENT_TIMESTAMP |更新CURRENT_TIMESTAMP | 
 + ------- + ----------- + ------ + ----- + ------------ ------- + ----------------------------- + 
 

Is there any possibility to use an "after update" trigger only in the case the data has been REALLY changed. I know of "NEW and OLD". But when using them I'm only able to compare columns. For example "NEW.count <> OLD.count".

But I want something like: run trigger if "NEW <> OLD"

An Example:

create table foo (a INT, b INT);
create table bar (a INT, b INT);

INSERT INTO foo VALUES(1,1);
INSERT INTO foo VALUES(2,2);
INSERT INTO foo VALUES(3,3);

CREATE TRIGGER ins_sum
    AFTER UPDATE ON foo
    FOR EACH ROW
    INSERT INTO bar VALUES(NEW.a, NEW.b);

UPDATE foo SET b = 3 WHERE a=3;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0


select * from bar;
+------+------+
| a    | b    |
+------+------+
|    3 |    3 |
+------+------+

The point is, there was an update, but nothing has changed. But the trigger ran anyway. IMHO there should be a way it doesn't.

I know that I could have used

IF NOW.b <> OLD.b

for this example.

BUT imagine a large table with changing columns. You have to compare every column and if the database changes you have to adjust the trigger. AND it doesn't "feel" good to compare every column of the row hardcoded :)

Addition

As you can see on the line

Rows matched: 1 Changed: 0 Warnings: 0

MySQL knows that the line didn't change. But it doesn't share this knowledge with the trigger. A trigger like "AFTER REAL UPDATE" or something like this would be cool.

As a workaround, you could use the timestamp (old and new) for checking though, that one is not updated when there are no changes to the row. (Possibly that is the source for confusion? Because that one is also called 'on update' but is not executed when no change occurs) Changes within one second will then not execute that part of the trigger, but in some cases that could be fine (like when you have an application that rejects fast changes anyway.)

For example, rather than

IF NEW.a <> OLD.a or NEW.b <> OLD.b /* etc, all the way to NEW.z <> OLD.z */ 
THEN  
  INSERT INTO bar (a, b) VALUES(NEW.a, NEW.b) ;
END IF

you could use

IF NEW.ts <> OLD.ts 
THEN  
  INSERT INTO bar (a, b) VALUES(NEW.a, NEW.b) ;
END IF

Then you don't have to change your trigger every time you update the scheme (the issue you mentioned in the question.)

EDIT: Added full example

create table foo (a INT, b INT, ts TIMESTAMP);
create table bar (a INT, b INT);

INSERT INTO foo (a,b) VALUES(1,1);
INSERT INTO foo (a,b) VALUES(2,2);
INSERT INTO foo (a,b) VALUES(3,3);

DELIMITER ///

CREATE TRIGGER ins_sum AFTER UPDATE ON foo
    FOR EACH ROW
    BEGIN
        IF NEW.ts <> OLD.ts THEN  
            INSERT INTO bar (a, b) VALUES(NEW.a, NEW.b);
        END IF;
    END;
///

DELIMITER ;

select * from foo;
+------+------+---------------------+
| a    | b    | ts                  |
+------+------+---------------------+
|    1 |    1 | 2011-06-14 09:29:46 |
|    2 |    2 | 2011-06-14 09:29:46 |
|    3 |    3 | 2011-06-14 09:29:46 |
+------+------+---------------------+
3 rows in set (0.00 sec)

-- UPDATE without change
UPDATE foo SET b = 3 WHERE a = 3;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0

-- the timestamo didnt change
select * from foo WHERE a = 3;
+------+------+---------------------+
| a    | b    | ts                  |
+------+------+---------------------+
|    3 |    3 | 2011-06-14 09:29:46 |
+------+------+---------------------+
1 rows in set (0.00 sec)

-- the trigger didn't run
select * from bar;
Empty set (0.00 sec)

-- UPDATE with change
UPDATE foo SET b = 4 WHERE a=3;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

-- the timestamp changed
select * from foo;
+------+------+---------------------+
| a    | b    | ts                  |
+------+------+---------------------+
|    1 |    1 | 2011-06-14 09:29:46 |
|    2 |    2 | 2011-06-14 09:29:46 |
|    3 |    4 | 2011-06-14 09:34:59 |
+------+------+---------------------+
3 rows in set (0.00 sec)

-- and the trigger ran
select * from bar;
+------+------+---------------------+
| a    | b    | ts                  |
+------+------+---------------------+
|    3 |    4 | 2011-06-14 09:34:59 |
+------+------+---------------------+
1 row in set (0.00 sec)

It is working because of mysql's behavior on handling timestamps. The time stamp is only updated if a change occured in the updates.

Documentation is here:
https://dev.mysql.com/doc/refman/5.7/en/timestamp-initialization.html

desc foo;
+-------+-----------+------+-----+-------------------+-----------------------------+
| Field | Type      | Null | Key | Default           | Extra                       |
+-------+-----------+------+-----+-------------------+-----------------------------+
| a     | int(11)   | YES  |     | NULL              |                             |
| b     | int(11)   | YES  |     | NULL              |                             |
| ts    | timestamp | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
+-------+-----------+------+-----+-------------------+-----------------------------+