How to create Triggers to add the change events into Audit Log tables

Question

Suppose we have 50 tables in a database and we want to capture all the changes (Previous value and new value of columns) across the columns of each table. An au

Answer 1

This is a little bit improved version of Vlad answer. Audit table has column for 'diff' of changes.

Auditing rules:

• INSERT and DELETE - complete record with all fields is stored to audit table
• UPDATE - only changes on fields is stored

Audit table structure. diff is column where changes are stored.

NB: privileges is JSON column on table we are auditing in this case.

CREATE TABLE roles_audit_log (
    roles_id INT UNSIGNED NOT NULL,
    diff JSON,
    dml_type ENUM('INSERT', 'UPDATE', 'DELETE') NOT NULL,
    dml_timestamp TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    dml_created_by VARCHAR(255) NOT NULL default 'system',
    PRIMARY KEY (roles_id, dml_type, dml_timestamp)
);

Trigger for UPDATE statement. Will fill diff column with fields that were changed.

DELIMITER $$

CREATE TRIGGER roles_audit_au AFTER UPDATE ON `roles` FOR EACH ROW
BEGIN
    DECLARE m_change text;
    SET m_change = JSON_OBJECT();

    IF coalesce(NEW.role, '') != coalesce(OLD.role, '') THEN
        SET m_change = JSON_SET(m_change, '$.role', NEW.role);
    END IF;

    IF coalesce(NEW.created_at, '') != coalesce(OLD.created_at, '') THEN
        SET m_change = JSON_SET(m_change, '$.created_at', NEW.created_at);
    END IF;

    IF coalesce(NEW.updated_at, '') != coalesce(OLD.updated_at, '') THEN
        SET m_change = JSON_SET(m_change, '$.updated_at', NEW.updated_at);
    END IF;

    IF coalesce(NEW.privileges, '') != coalesce(OLD.privileges, '') THEN
        SET m_change = JSON_SET(m_change, '$.privileges', NEW.privileges);
    END IF;

    INSERT INTO `roles_audit_log` (
        roles_id,
        diff,
        dml_type,
        dml_created_by
    ) VALUES(
        NEW.id,
        m_change,
        'UPDATE',
        coalesce(@logged_user, 'system')
    );
END;$$
DELIMITER ;

Trigger for DELETE statement. Will fill diff with complete row that was deleted.

DELIMITER $$

CREATE TRIGGER roles_audit_ad AFTER DELETE ON `roles` FOR EACH ROW
BEGIN
    INSERT INTO `roles_audit_log` (
        roles_id,
        diff,
        dml_type,
        dml_created_by
    ) VALUES(
        OLD.id,
        JSON_OBJECT(
            'id', OLD.id,
            'role', OLD.role,
            'privileges', OLD.privileges,
            'created_at', OLD.created_at,
            'updated_at', OLD.updated_at
        ),
        'DELETE',
        coalesce(@logged_user, 'system')
    );
END;$$
DELIMITER ;

Trigger for INSERT statement. Will fill diff with complete row that was deleted.

DELIMITER $$

CREATE TRIGGER roles_audit_ai AFTER INSERT ON `roles` FOR EACH ROW
BEGIN
    INSERT INTO `roles_audit_log` (
        roles_id,
        diff,
        dml_type,
        dml_created_by
    ) VALUES(
        NEW.id,
        JSON_OBJECT(
            'id', NEW.id,
            'role', NEW.role,
            'privileges', NEW.privileges,
            'created_at', NEW.created_at,
            'updated_at', NEW.updated_at
        ),
        'INSERT',
        coalesce(@logged_user, 'system')
    );
END;$$
DELIMITER ;