Databases Illuminated, 4th Edition

3.11 Union

Whereas a shared subset, as described in the previous section, is a member of all of its supersets and inherits attributes from all of them, a subset can instead be related to just one member of a collection, called a union or category, of supersets, rather than belonging to all of them. In that case, an instance of the subset inherits the attributes of only one of the supersets, depending on which member of the union that it belongs to.

Expanding on our University example, suppose we want to represent sponsors of campus-wide events, which can be sponsored by teams, departments, or clubs. We assume here that a campus-wide event must have exactly one sponsor, but it can be a team, a department, or a club. We could form a union of ...