For Example, consider the binary relationship given in the figure 1 involving two zero, then the FK is defined as normal and it can have the Null value, on the. I'm not totally sure where your definition of "strong" vs. "weak" originates from . For example, the relationship "is married to" would be unary, but not recursive. two entity types and is referred to as a binary relationship. Below is a ternary In the ERD we define a . In this example, Section is modeled as a Weak Entity.

N-ary relationship types

It may or may not have a FK. Thus a relationship may be weak breakable or strong unbreakable. A strong relationship just means that the dependent entity cannot exist apart from the relationship. Take Class, Course and Room, for example. Imagine the following conversation: What course will you teach?

Since a course must be specified before the class is created, the relationship is a strong one. Also, when considering which class to take, a class in "Bead Rendering" and a class in "Cybercrafting" are two completely different things even though they may both meet in the same room at the same time just on different days. The identity of each class is inexorably bound to a course.

So this relationship is also identifying.

What room will it be in? Yes, a room will have to be assigned before the semester starts, but at the moment we can still create the class, put "TBD" in the catalog and allow students to enroll.

The class can exist without a room for a while, anyway so the relationship is weak.

Unary Relationships (recursive)

Also, two classes in "Discretionary Logic" are functionally equivalent, even though they are taught in different rooms. The relationship with the room has nothing to do with the type of class. The relationship is non-identifying. So if students had signed up to take Bead Rendering in room 17 were notified that the room had changed to 12, they would not think, "This is a completely different class!

However, if they were told the class was now "Second-hand Carnival Staffing" then they would be right.

This is now a completely different class. This is the difference between identifying and non-identifying relationships.

Unary Relationships (recursive) | Open Textbooks for Hong Kong

It's important to realize that all relationships consist of at least two entities. In this way a database relationship is similar to a human relationship -- it takes two to Tango. The "unary" just means that both sides of the relationship are filled by the same logical entity. It is easy to see that a "meets in" relationship between a Class entity and a Room entity cannot be satisfied between, say, two Class entities or two Room entities.

However, a "is managed by" relationship requires an Employee entity on both sides.

It should also be obvious that an employee does not require this relationship in order to exist. It helps us understand what kind of information we want to store and what kind of relationships there are.

It is imperative that this diagram is easy to read and understand. The number of entities in a relationship is the arity of this relationship. The aim of this article is to give some examples and show how big an impact the arity of relationships has on not only the readability of the diagram, but also the database itself.

The most common types of relationships are: Unary one entity is invloved in the relationship. Binary two entities are involved in the relationship. Unary relationship type A Unary relationship between entities in a single entity type is presented on the picture below.

As we see, a person can be in the relationship with another person, such as: This is definetly the most used relationship type. Journalist writes an article. This example can be implemented very easily. In the diagram below, we represent our ternary relationship with an extra table, which can be modelled in Vertabelo very quickly.