a$)$ In the EER model, a superclass$/$subclass relationship allows a subclass to inherit attributes and relationships from its superclass. This means that the subclass will have all the attributes and relationships of the superclass, in addition to any attributes and relationships specific to the subclass. On the other hand, a category $($or union type$)$ represents a collection of entities from different entity types $($superclasses$)$ that share some common attributes. In a category, the entities do not inherit attributes or relationships from the superclasses; instead, the category itself defines the common attributes.

$($b$)$ When mapping superclass$/$subclass relationships with overlapping constraints to relations using the approach of multiple relations for subclass relations only, one drawback is redundancy. Since each subclass relation must include the primary key of the superclass, the same entity may be represented multiple times across different subclass relations, leading to data redundancy and potential inconsistencies. Additionally, querying the data can become more complex and less efficient due to the need to join multiple relations to retrieve complete information about an entity.

$($c$)$ To derive the primary key of the relation mapping from a weak entity type, you need to combine the primary key of its identifying $($owner$)$ entity type with the partial key of the weak entity type. The primary key of the weak entity type's relation is formed by concatenating these keys.

For example, if the identifying entity type has a primary key

and the weak entity type has a partial key

$,$ the primary key of the weak entity type's relation is: