How to know if there is a record in the database using codeignaiter 3

Hi, I'm new to code-code and I wanted to know if they could help me with something I still do not understand, and it's like knowing if there is a record in the database and showing an alert saying that that record already exists in the database, please If someone could help me, it's a university project and I have two days to get it and I have 2 weeks trying to do that, please if someone can help me.