Problem
Hiroshi Yoshimura is chair of the Panda Preservation Project, which maintains a database of all panda bears across the globe. The database design includes a relational database comprising two tables. The first table stores the name, birth date, and other characteristics of panda bear parents that have been registered. In this table, each panda bear parent is uniquely identified by a registration number.
The second table stores the name, birth date, and other characteristics of panda bear kids that have been registered. In this table, each panda bear kid is uniquely identified by a registration number. This second table also includes the registration numbers of each panda bear kid's parents. The panda bear kids in the second table are linked to their parents in the first table by means of the registration numbers of both parents.
How many primary keys are in the relational database?