Constantine Kokkinos provides an explanation:
I was talking to a friend of mine and they are learning some SQL and they said something that I have seen come up multiple times in learning SQL.
They said “Yeah, I need to study the join types more. They make sense to me but I want to be able to not reference my notes” and also “I don’t really get the point of a right join if your can do the same thing with a left join by just switching the table name.”
These are great points, and common questions that occur when first learning SQL.
I won’t steal CK’s thunder (too much) about how we express joins in set theory, though I think when he mentions “OUTER” as a type of join, perhaps that’s supposed to be FULL OUTER JOIN?
Regardless, my take: there is a good reason to use INNER JOIN. There is a good reason to use LEFT OUTER JOIN. There is a good reason to use CROSS JOIN. There is a good reason to use FULL OUTER JOIN. The frequency in which you should use each is in descending order, meaning that there are relatively few circumstances in which you should use a FULL OUTER JOIN, but they do exist.
There are no good circumstances for a RIGHT OUTER JOIN. The concept logically exists, but has no practical value to us.