为什么我们不能通过交换表而不是右外连接来使用左外连接？

few days back, I have faced the question in an interview as following. "what is Right outer join?" I answered, " Right outer join joins two tables and returns the matched records from both the tables as well as non matching rows from Table A". the interviewer simply laughed and asked again, "Why we use Right join rather we can swap the tables and use left join?". Then my answer was," yes, we can use". But, he argued that "then why the concept of two joins? (left and right)" . I really got confused with this question. Please help me on this and correct my understanding on joins. few days back, I have faced the question in an