In ER models there are entity types without key attributes for identification. These are called weak entity types. In contrast Entity types that do have a key attribute are called strong entity types.

Generally in any entity type we have lots of entities. To distinguish them we have key attributes to satisfy uniqueness constraint. For example, in Employee entity type in an organization the entity set will contains details of all employees. To identify each employee organizations usually assign unique employee id to each employee. This employee id will be the key attribute for Employee Entity Type. Since it uniquely identifies each employee, it becomes a Primary Key.

Sometimes an entity type may have more than one attribute that qualify for key attributes. Sometimes designers are unable to identify an attribute to be defined as key attribute. Due to lack of key attributes for some Entity Types designers can face serious design issues.

For example, consider an entity type Dependent which is used to keep record of dependents of employees of an organization. The attributes of Dependent entity type are Dependent Name, Birth Date, Gender and Relationship (with the employee). In this entity Type if Name is identified as the discrimination attribute then it may be possible that two employees may have dependents whose attributes Dependent Name, Birth Date, Gender and Relationship are same. They are distinct entities but just on the basis of Dependent Name it is impossible to identify that which employee a dependent is related to.  To overcome this issue such dependents can  identified as distinct entities only after associating them with a particular employee entity to which each dependent is related.

To handle the problem of uniqueness in weak entities, every weak entity must be related to a strong entity in a relationship. That relationship is called Identifying Relationship. This relationship is mandatory in case an ER model contains the Strong and Weak Entities. A Weak Entity has to depend on a strong entity for its existence. So, it creates a total participation in the identifying relationship. A weak entity has existential dependency on its owner entity.  A Strong Entity can have a partial participation in the Identifying Relationship.

In a weak Entity the Primark Key includes the key attribute of the related Strong Entity along with its own discriminating attribute. In the example Dependent Entity Type Primary Key is {Employee ID, Dependent Name}

Strong and Weak Entity