Strong Entity Set | Weak Entity Set |
Strong entity set always has a primary key. | It does not have enough attributes to build a primary key. |
It is represented by a rectangle symbol. | It is represented by a double rectangle symbol. |
It contains a Primary key represented by the underline symbol. | It contains a Partial Key which is represented by a dashed underline symbol. |
The member of a strong entity set is called as dominant entity set. | The member of a weak entity set called as a subordinate entity set. |
Primary Key is one of its attributes which helps to identify its member. | In a weak entity set, it is a combination of primary key and partial key of the strong entity set. |
In the ER diagram the relationship between two strong entity set shown by using a diamond symbol. | The relationship between one strong and a weak entity set shown by using the double diamond symbol. |
The connecting line of the strong entity set with the relationship is single. | The line connecting the weak entity set for identifying relationship is double. |
Types of Attributes | Description |
Simple attribute | Simple attributes can't be divided any further. For example, a student's contact number. It is also called an atomic value. |
Composite attribute | It is possible to break down composite attribute. For example, a student's full name may be further divided into first name, second name, and last name. |
Derived attribute | This type of attribute does not include in the physical database. However, their values are derived from other attributes present in the database. For example, age should not be stored directly. Instead, it should be derived from the DOB of that employee. |
Multivalued attribute | Multivalued attributes can have more than one values. For example, a student can have more than one mobile number, email address, etc. |
Entity | Primary Key | Attribute |
Student | Student_ID | StudentName |
Professor | Employee_ID | ProfessorName |
Course | Course_ID | CourseName |