Many-to-Many relationships needing an associate table

Question

In many database design tutorials/articles, they always bring up the fact that if two tables share a many-to-many relationship, then a third table should be created to act as an

Answer 1

In relational databases all relationships are represented in only one way: as relations (relations correspond to tables in SQL). A relation with two attributes, such as R{A,B}, represents a binary relationship between A and B. That relationship could be one-to-many or many-to-many for example.

If the relationship represented by R{A,B} is many-to-many that implies that neither A or B are candidate keys (because if either was unique then obviously only ONE tuple for each value of that attribute would be permitted). That means that the principle of Third Normal Form requires any attributes dependent on A or B to go in other tables. The reason for this is that non-key dependencies (attributes dependent on A or B) are a form of redundancy and can cause anomalies and incorrect results.

So it's not that "many-to-many relationships" are represented any differently to other relationships. It's just that normalization often leads to a common pattern with tables with compound keys and no other non-key attributes. Some people like to call that pattern an Association table - although personally I don't find that terminology very helpful.