WEAK ENTITY AND STRONG ENTITY
Strong Entity Set:
A strong entity set is an entity set that has a primary key attribute, which uniquely identifies each entity within the set.
The primary key is a minimal set of attributes that can uniquely identify an entity.
Strong entity sets do not depend on the existence of another entity set for their identification.
Example: In a university database, the "Student" entity set may be considered a strong entity set because each student can be uniquely identified by a student ID.
Weak Entity Set:
A weak entity set is an entity set that does not have a primary key attribute on its own.
The existence of entities in a weak entity set is dependent on the existence of another entity set, known as the owner or parent entity set.
Weak entities are typically identified by a partial key, which is a set of attributes that only uniquely identifies entities when used in conjunction with the owner entity's primary key.
Example: Consider a "Dependent" entity set that depends on a "Employee" entity set. The "Dependent" entity set might have attributes like pname, but to uniquely identify a Dependents, you need both the pname and the Employee.