Weak Entity Sets
An entity set that does not have a primary key is referred to as a weak entity set.
The existance of a weak entity set depends on the existence of a strong entity set; it must relate to the strong set via a on-to-many relationship set.
A distriminator of a weak entity set is the set of attributes that distinguishes among all the entities of a weak entity set.
The primary key of a weak entity set is formed by the primary key of the strong entity set on which the weak entity set is existence depend, plus the weak entity set’s discriminator.