如何从一个mysql表中获得不同的结果？

I have a user table in the database where all users of the system are stored. The table has a user_id and a business_name and a first_name. Some users are merchants and get a business name, some users are consumers and get a first name. In a second table I have transactions with a user_id and a merchant_id (which are defining the transaction) and an amount. Both ids reference to user table.I have a user table in the database where all u