... default candidate key to be used to select particular rows of the table
Can anyone expand on what is meant by 'default' here?
In addition to what's been said: In SQL, you can define a foreign key by specifying only the table it relates to, leaving out the key. If you do that, the primary key is chosen from the possible candidate keys. There is some debate among relational theorists as to whether a properly normalised table will have more than one candidate key but, heck, we've all seen them, hence the usefulness of having a primary key.