2. What are weak entity sets? Give an example where weak entity sets would be necessary (not the examples given in class think of your own). [10]

3. What is the minimum possible number of attributes of a key for an entity set E? Then what is the maximum? [2]

4. We think of relationships in the ER model as having keys, just as entity sets do. Let R be a relationship between entity sets E1 and E2, where K1 and K2 are the set of attributes that is a key for entity set E1 and E2, respectively. What is the minimum possible number of attributes of a key for R (in terms of K1 and K2) under the assumption that: (a) R is many-many [2]; (b) R is many-one from E1 to E2 [2]; (c) R is many-one from E2 to E1 [2]; (d) R is one-one [2];