Mockito: Moacking the database interactions

I am new to mockito. I am trying to test a function which has a functionality that it registers a username. So before updating the new username, it checks if that username is already registered(present in DB) or not. So here I have to test two cases: 1. Success: The username registration successful.(No username with given name in DB found)

2. Failure: Failed to register as the username is already registered.

I am not able to understand how to mock the functions. The functions signature is as follows:

func registerName(User){
     cond = checkUsername(User);
     if(cond) return false; //since user is found
     else CompleteRegistration(User);}

Can someone help me to mock the function. Also the checkUsername() and CompleteRegistration() only have basic DB interactions.