Why is it an INNER JOIN, not a LEFT OUTER JOIN, because if a car doesn't have a make I still want to show it?

This is a bit of a guess.

Since you didn't say which challenge, or which task, I'm taking a guess here. But if you're talking about the challenge which says "For all cars in the database, show Model Name, VIN and Sticker Price in one result set." reasons that an INNER JOIN might be expected:

• they did not explicily ask for cars without ModelId's to be included
• they only want rows with complete data
• every car might be required to have a ModelId (it might be a non-NULL field)

However, you might still want to report this as a bug to Support, it might get you a "special Exterminator badge" and/or get the wording of the question changed to make the requirements more clear.

I was thinking along the lines of Steven Parker, the cars are probably required to have a ModelID which means an inner join will still return all cars.