MySQL Trigger after update only if row has changed

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 |
+-------+-----------+------+-----+-------------------+-----------------------------+

> 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 row hardcoded :)

Yeah, but that's the way to proceed.

As a side note, it's also good practice to pre-emptively check before updating:

UPDATE foo SET b = 3 WHERE a=3 and b <> 3;

In your example this would make it update (and thus overwrite) two rows instead of three.

I cant comment, so just beware, that if your column supports NULL values, OLD.x<>NEW.x isnt enough, because

SELECT IF(1<>NULL,1,0)

returns 0 as same as

NULL<>NULL 1<>NULL 0<>NULL 'AAA'<>NULL

So it will not track changes FROM and TO NULL

The correct way in this scenario is

((OLD.x IS NULL AND NEW.x IS NOT NULL) OR (OLD.x IS NOT NULL AND NEW.x IS NULL) OR (OLD.x<>NEW.x))

You can do this by comparing each field using the NULL-safe equals operator <=> and then negating the result using NOT.

The complete trigger would become:

DROP TRIGGER IF EXISTS `my_trigger_name`;

DELIMITER $$

CREATE TRIGGER `my_trigger_name` AFTER UPDATE ON `my_table_name` FOR EACH ROW 
	BEGIN
		/*Add any fields you want to compare here*/
		IF !(OLD.a <=> NEW.a AND OLD.b <=> NEW.b) THEN
			INSERT INTO `my_other_table` (
				`a`,
				 `b`
			) VALUES (
				NEW.`a`,
				NEW.`b`
			);
		END IF;
	END;$$
   
DELIMITER ;

(Based on a different answer of mine.)

In here if there any row affect with new insertion Then it will update on different table in the database.

DELIMITER $$

CREATE TRIGGER "give trigger name" AFTER INSERT ON "table name" 
FOR EACH ROW
BEGIN
    INSERT INTO "give table name you want to add the new insertion on previously given table" (id,name,age) VALUES (10,"sumith",24);
END;
$$
DELIMITER ;

Use the following query to see which rows have changes:

(select * from inserted) except (select * from deleted)

The results of this query should consist of all the new records that are different from the old ones.

MYSQL TRIGGER BEFORE UPDATE IF OLD.a<>NEW.b

USE `pdvsa_ent_aycg`;

DELIMITER $$

CREATE TRIGGER `cisterna_BUPD` BEFORE UPDATE ON `cisterna` FOR EACH ROW

BEGIN

IF OLD.id_cisterna_estado<>NEW.id_cisterna_estado OR OLD.observacion_cisterna_estado<>NEW.observacion_cisterna_estado OR OLD.fecha_cisterna_estado<>NEW.fecha_cisterna_estado

	THEN 

		INSERT INTO cisterna_estado_modificaciones(nro_cisterna_estado, id_cisterna_estado, observacion_cisterna_estado, fecha_cisterna_estado) values (NULL, OLD.id_cisterna_estado, OLD.observacion_cisterna_estado, OLD.fecha_cisterna_estado); 

	END IF;

END

Here are two interesting dead ends (as of MySQL 5.7)-

1. The new.* and old.* constructs are invalid, MySQL complains about Unknown table 'new' or syntax to use near '*, which precludes tricks like select ... from (select (select new.* union select old.*)a having count(*)=2) has_change

2. The documentation for "ROW_COUNT()" has a useful clue-

> For UPDATE statements, the affected-rows value by default is the number of rows actually changed

And indeed, after an update statement, ROW_COUNT() correctly shows the count of rows that had changes from the update. However, during the update, inside the trigger, ROW_COUNT() = 0 always. That function has no useful value in a row-level trigger, and there's no statement-level trigger in MySQL as of this answer.

Hope this "null result" prevents future frustration.