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Find Which Columns Have Changed In MySQl Triggers

- 1 answer

I am working on an audit requirement for a legacy application. The legacy application was written in pure java and jdbc with underlying DB in My SQL. I need to populate an audit table with any changes for a given table, the audit table DDL is something like this

CREATE TABLE ENTITY_AUD (
    id INT AUTO_INCREMENT PRIMARY KEY,
    baseTableId INT FK,
    beforeValue INT,
    afterValue INT,
    changedAt TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    changedBy VARCHAR2
);

I have to do this via triggers, since we want to avoid any changes to the legacy java code. I know when can can know the old and new values of a given column by using the old and new variables, something like this

DELIMITER $$

CREATE TRIGGER ENTITY_AFTER_UPDATE
AFTER UPDATE
ON Entity FOR EACH ROW
BEGIN
    IF OLD.quantity <> new.quantity THEN
        INSERT INTO ENTITY_AUD(baseTableId,beforeValue, afterValue,changedBy)
        VALUES(old.id, old.quantity, new.quantity,new.updated_by);
    END IF;
END$$

DELIMITER ;

How can i dynamically find all the columns which have changed and populate new rows for each column updates? i do not want If statements for each column, the source tables may have 40 columns

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Answer

It would be best to use a CDC tool like Debezium for this instead of triggers.

You can get a list of columns belonging to your Entity table by querying INFORMATION_SCHEMA.COLUMNS, but that doesn't help. It's not possible to generate dynamic statements in a trigger based on the columns, because MySQL doesn't support running dynamic SQL in a trigger.

If you must use triggers, then you just have to hard-code all 40 columns.

You could streamline it a little bit, by using the new feature in MySQL 8.0 feature to add a WHERE clause to a SELECT without a table reference. If the WHERE clause is false, the SELECT returns zero rows. Then you can string these together with UNION to make a set of rows, one for each column that changed.

BEGIN
    INSERT INTO ENTITY_AUD (baseTableId, beforeValue, afterValue, changedBy)
    SELECT OLD.id, OLD.quantity, NEW.quantity, NEW.updated_by
    WHERE OLD.quantity <> NEW.quantity
    UNION 
    SELECT OLD.id, OLD.price, NEW.price, NEW.updated_by
    WHERE OLD.price <> NEW.price
    UNION
    SELECT OLD.id, OLD.billing, NEW.billing, NEW.updated_by
    WHERE OLD.billing <> NEW.billing
    UNION
    ...37 others...
END

This question has come up numerous times in the past. Here are a few I found, but undoubtedly there are more.

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source: stackoverflow.com
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