Hi. I'm finding some difficulties in mapping an ER scheme into a relational scheme. The structure I'm trying to map is a relationship of third grade. I have three entities and the cardinal constraints are (0,1), (1,1) and (0,N), where the first number indicates the minimum number of istances of the relation and the second one the maximum. I mapped this situation adding a fourth relation which maps the relationship, which has three columns, which are the primary keys of the relations I'm trying to map. The problem is: which will be the primary key of the new relation? Which one of those three columns?
Thanks.

Since you have a 1:1 relationship in there then the PK of the relation at one end can serve as the PK of the relation representing the relationship.