Weak Entity Types
Entity types
An entity type is a grouping of entities that share common characteristics. A corporate company's database, for example, contains entity types like departments, employees, and so on. Each entity type in a DBMS has a collection of characteristics that describe the entity.
Name, age, address, phone number, and income are examples of properties that can be included in the Employee entity type.
Name, number, and departmental location are examples of properties that can be assigned to the department entity type.
Types of Entity Types
There are two kinds of Entity types
1. Strong Entity Type
2. Weak Entity Type
Weak Entity Type
No Key Attribute is present in a Weak Entity in DBMS. To be uniquely identifiable, it must be connected to another powerful entity.
The other explanation of weak entity: It is a creature that depends on a powerful entity to sustain it rather than having an existence of its own. Double rectangles are used in the Entity Relationship Diagram to represent the weak entity type
Weak entity types are those which do not have essential characteristics of their own. Being related to particular entities from another entity type combined with one of its attribute values identifies entities that belong to a weak entity type. This additional entity type is known as the owner or identifying entity type. We refer to the connection type that connects a weak entity type to its owner as the weak entity type's identifying relationship.
Consider the entity type DEPENDENT, which is associated with EMPLOYEE and is used to maintain a 1: N relationship with each employee's dependents. Name, birthdate, gender, and relationship (to the employee) are the characteristics of a dependent in our example. Two dependents of two unique employees may, by chance, have the same values for Name, Birth_date, gender, and Relationship, but they are still distinct entities. Only after identifying the specific employee entity to whom each dependent is tied are they recognized as separate entities. It is believed that each employee entity owns the connected dependent entities.
Since a weak entity cannot be identified without an owner entity, it always has a whole participation restriction (presence dependency) with regard to its identifying connection. The property that can uniquely identify weak entities that are associated to the same owner entity is called a partial key, and it is typically included in weak entity types. The attribute Name of DEPENDENT is the partial key in our case, assuming that no two dependents of the same employee ever share the same first name.
