if i was describing lets say classes and students where each student can be in more than one class and each class can have multiple students. Or how about just using Table A by itself. I'm sorry, I'm new to SQL.
Table A ID Student Class 1 Bob math 2 Tim math 3 Linda english 4 Linda science
You can certainly model a one to many with one table, but I wouldn't do it for a number of reasons. The point is that, in general, normalization is a good thing. If you don't use a junction table, then your leaving you data less normalized. If you are not familiar with normalization, then you should look it up! That is one of the fundamental foundations of relational theory. :)