Table trigger to indicate two records were merged - mysql

Question

I have some simple triggers on a table which work for to audit changes to a record by logging to a table any time an INSERT, UPDATE, or DELETE occurs. When a record is inserted into Users, a copy is made in Users_History with a Status field populated with the value 'INSERT'. Same is done for UPDATE and DELETE.
DELIMITER $$
CREATE TRIGGER Users_History_Insert AFTER INSERT ON Audit
FOR EACH ROW BEGIN
INSERT INTO Users_History select *, 'INSERT' from Users where ID = NEW.ID;
END$$
DELIMITER ;
DELIMITER $$
CREATE TRIGGER Users_History_Delete BEFORE DELETE ON Audit
FOR EACH ROW BEGIN
INSERT INTO Users_History select *, 'DELETE' from Users where ID = OLD.ID;
END$$
DELIMITER ;
DELIMITER $$
CREATE TRIGGER Users_History_Update AFTER UPDATE ON Audit
FOR EACH ROW BEGIN
INSERT INTO Users_History select *, 'UPDATE' from Users where ID = NEW.ID;
END$$
DELIMITER ;
We have a function where two Users records can be merged with each other. Effectively, all values of one record are overwritten with the values of another record, and the unchanged record is deleted.
If you had a table such as this:
| ID | Name | email |
| 1 | Billy | bill#mail.com |
| 2 | Bill | bill2#mail.com |
And I wanted to merge record 2 into 1, I'd end up with this result.
| ID | Name | email |
| 1 | Bill | bill2#mail.com |
I'd like to create a trigger that would have this in Users_History where 2 is the record merged from.
| Users_ID | Name | Email | Status |
| 1 | Bill | bill2#mail.com | MERGED:2 |
As it stands now, we would end up with two records, an UPDATE and a DELETE
UPDATE `Users_To`
SET `Name` = `Users_From`.`Name`, `email` = `Users_From`.`email`
FROM `Users` AS `Users_To`
CROSS JOIN `Users` AS `Users_From`
WHERE `Users_To`.`ID` = 1 AND `Users_From`.`ID` = 2;
DELETE FROM `Users` WHERE `ID` = 2;
I can't think of a way to do this in SQL, is it possible? As I understand, the triggers have no knowledge of the JOIN or WHERE clause affecting a record.

Answer 1

Your UPDATE query has a flaw and is correctly
UPDATE `Users` AS `Users_To` CROSS JOIN `Users` AS `Users_From`
SET `Users_To`.`Name` = `Users_From`.`Name`, `Users_To`.`email` = `Users_From`.`email`
WHERE `Users_To`.`ID` = 1 AND `Users_From`.`ID` = 2;
DELETE FROM `Users` WHERE `ID` = 2;
What the trigger does is simple first it check if the update is a modify of an existing row by checking if there are mor than one rows with the same email.
If that the case grab the other id in your case 2 and add it ti the text.
This works as long you have only 2 rows involved in your update query.
And i don't know your your table, but if you have a UNIQUE constraint you will have a problem with the update.
DELIMITER $$
DROP TRIGGER IF EXISTS Users_History_Update;
CREATE TRIGGER Users_History_Update AFTER UPDATE ON users
FOR EACH ROW BEGIN
SELECT Count(*) INTO #number FROM Users WHERE `email` = NEW.`email`;
IF #number > 1 THEN
SELECT `ID` INTO #id FROM Users WHERE `email` = NEW.`email` AND `ID` <> NEW.`ID`;
INSERT INTO Users_History VALUES (OLD.`ID`,OLD.`Name` ,OLD.`email`, CONCAT('MERGED:',#id));
ELSE
INSERT INTO Users_History VALUES (OLD.`ID`,OLD.`Name` ,OLD.`email`, 'UPDATE');
END IF;
END$$
DELIMITER ;