table join

Hi everyone I'm having a whole lot of fun playing with this project. It just keeps expanding and taking me to new territory and I'm really enjoying it. I got a quick question though. I've been playing around with table joins and I tried to join based on the ID but I quickly realized this is not a good solutions, because the ID's in each table aren't always going to match. This is a simple join that I was messing around with: $sql = 'SELECT * FROM inventory, extended_info WHERE inventory.id = extended_info.id'; So I'm wondering if I should figure a way to always make my ID's in each table match, which is probably not possible or I'm thinking a unique field for both tables and use that to match up the records, but I know zilch about cars (funny considering what I chose to start with) and I couldn't think of 1 thing that would be unique to match up... LOL so what would be a wise way to go about this type of thing. PS I'm not looking for code, just some feedback on logic. -- Regards David M.