[mysql] 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.

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.)

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.

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

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))

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.