Learn Computer Science

ERD Diagrams

Tutoring > Databases > ERD Diagrams

There are different types of entity relationship diagram (ERD) used to model a database design. Most notable are the Chen and Crow's Foot diagram styles. We will look at each in turn, there are similarites, but also subtle differences between them.

 

Chen Diagrams

 

​Peter Chen's notation is very popular due to it's ease of use and clear design. Straight lines connect entities with relationships and attributes.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Crow's Foot Diagrams

 

Crow's Foot style diagrams are a bit more detailed than Chen and show cardinality with the connecting lines (Cardinality specifies how many instances of an entity relate to one instance of another entity). 

• The attributes are listed inside the boxes along with their entities.

• The Primary Key is labelled and in bold at the top of the attribute list.

• Relationships are optional if they are not absolutely required. Otherwise they are mandatory.

• Please see Entities and Relationships for more on relationships.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Generalisation hierarchy

A generalisation hierarchy shows the levels of a database (or part of one).

For example, if we had animals in a database, we could say that a lion is a mammal, and a mammal is an animal.

 

 

Like this:

 

 

Notice that the super type is animal.

It's sub type is Mammal.

The two sub types of mammal are Lion and gazelle.

 

These seperate levels have an IS A relationship.

 

A lion IS A Mammal

A Gazelle IS A Mammal

A Mammal IS AN Animal

 

 

 

 

 

We can also specify if a relationship of this type is disjoint or overlapping.

 

• In a disjoint specialization, also called an exclusive specialization, an individual of the parent class may be a member of only one specialized subclass.

• In an overlapping specialization, an individual of of the parent class may be a member of more than one of the specialized subclasses.