Database triggers aren't really in the domain of Appian functionality / knowledge. Assuming you are on Appian cloud / MariaDB - here is a good place to start https://mariadb.com/kb/en/trigger-overview/
I completely agree with this, but I'd also be curious why you're planning on creating a database trigger. There are certainly valid use cases for them, but there may also be out-of-the-box functionality in Appian that could meet your use case too.