Final answer:
A key in a weak entity set is composed of attributes that are derived partially or entirely from another entity set. Weak entity sets have partial keys and rely on foreign keys linking them to strong entity sets for unique identification.
Step-by-step explanation:
In weak entity sets, a key is composed of attributes, some or all of which belong to another entity set. Weak entity sets do not have sufficient attributes to form a primary key on their own and rely on being related to other entity sets (usually a strong entity set) from which they can derive the additional attributes needed for a complete key. This dependency results in the weak entity set having a partial key, which is its unique identifier within the scope of the strong entity set, and a foreign key, which links it to the strong entity set.
An example could be a situation in which we have an entity set for 'Books' (a strong entity set with a primary key of ISBN) and a related 'Chapters' weak entity set (which does not have a primary key on its own). Here, 'Chapters' might have a partial key like 'Chapter Number' which only uniquely identifies it in combination with the 'Book' it is part of, connected through the ISBN as a foreign key.