这些是增强的关系。子类和超级类的概念来定义这些关系。基于一些专门的属性集的实体可以被组织为一个子类，并且基于一些通用的属性集可能被组织为一个超类。例如，根据一些数据库必需品将实体“电影”划分为实体“电影1”，包括电影名称，电影_年份，电影_发行日期和电影 – 名称，电影_语言，电影_分类等实体’电影2’。然后这两个实体具有非常相似的属性，因此可以推广为实体’电影’。
The primary key is a unique attribute or identifier for the entity. In the case of the entity Cinema, the primary key would be the registration number. Similarly in the case of the employee, the primary key would be the Social Security Number SSN.
Strong and Weak Entity
An entity that has a sufficient set of attributes to stand alone and be defined with a primary key is a strong entity. The Cinema and Employee are strong entities. A weak entity is one which lacks a primary key and may be used in conjunction with another entity. Entity InQueue_Booking could be a weak entity.
Two or more entities will relate to one another by means of action-verbs. This can be a one-to-many, many-to-many or a one-to-one relationships. An Example of a one to much relationship is an Entity ‘Cinema’ has Entity ‘Cinema_Screen’. The Entity ‘Cinema’ refers to one location of Winterfell cinemas that has many screens in it (Screen 1, 2..X).
Generalization and Specialization Relationships
These are enhanced relationships. The concept of subclass and super class comes into the picture to define these relationships. Entities based on some specialized set of attributes may be organized as a sub class and based on some general set of attributes might be organized as a superclass. For instance, Entity ‘Movie’ was divided based on some database necessities into Entity ‘Movie1’ including attributes Movie_Name, Movie_Year, Movie_Release Date, and Entity ‘Movie2’ including Movie-Name, Movie_Language, Movie_Classification. Then these two entities have quite similar attributes and hence can be generalized to be taken as Entity ‘Movie’.