The Spouse table, in the COMPANY database, is a weak entity because its primary key is dependent on the Employee table.Its primary key is derived from the primary key of the parent entity.That is, it cannot exist without a relationship with another entity.An entity is considered weak if its tables are existence dependent. An object with conceptual existence (e.g., a course, a job, a position)Įntities can be classified based on their strength.An object with physical existence (e.g., a lecturer, a student, a car).Each dependent has a name, birthdate and relationship with the employee.Īn entity is an object in the real world with an independent existence that can be differentiated from other objects. We want to keep track of the dependents for each employee.
![parallel query processing in dbms parallel query processing in dbms](https://s3.manualzz.com/store/data/021495524_1-e514b98c0e7414fa7a65bccc640c94f0.png)
We also need to know the direct supervisor of each employee. We need to record the start date of the employee in each project. An employee is assigned to one department but can join in several projects.
![parallel query processing in dbms parallel query processing in dbms](https://i.imgur.com/0eKjitA.png)
Chapter 8 The Entity Relationship Data Model