May 18, 2014 by Ozgur Ozden
Entity relationship diagram known as ERD is a visual or graphical representation of the entities contained in a database and illustrates its relationship between them. Before we go into details I would like to provide brief information about ERD notations and the common language used in these notations. It is better to start with four basic terms namely Entity, Attribute, Relationship and Cardinality.
Entity might be a place, person, object or a thing which will be stored in a database. If we think about a school database entity might be STUDENT, TEACHER, COURSE.
Attribute is the property or characteristic of the entity. For example if we think about the entity STUDENT, attribute might be name, lastname, id no etc.
Relationship is the link between the entities and how they collaborate with each other. If we take the entities STUDENT and COURSE the obvious relationship between them is taking a course. So STUDENT takes COURSE. But how many?
Cardinality is the answer to this question. Tables, entities might be connected to each other by three different relationships as one-to-one, one-to-many and many-to many. For example, a STUDENT may take MANY COURSES, a STUDENT must have ONE STUDENT ID etc…
Before we start comparing the Chen and Crow’s notations i would like to briefly mention about these notions so we can have a better understanding.
It is first introduced by Peter Chen in 1976 as a way to visualize the entity, relationship and attribute by means of geometrical shapes as shown below.
In the chen model definition of an entity and attribute is same as we have described above and relationship between the entities are described as below.
Crow’s notation is another method to illustrate the relationship between the entities. Crow’s notation uses boxes which repreuıosents entities and uses lines to describe the relationships between the entities. Below diagram on the left represents a typical crow’s foot notation and the one on the right represents the relationships between them.
Both notations have advantages and disadvantages and pros and cons can be explained as in the below.
Some people think that chen notation takes too much space so this is a disadvantage for chen notation Coronel (2011).
Symbols used in the Chen notation might confuse the end users.
Crow’s feet is intuitive because when we try to explain things we always use boxes and lines so it is natural for us to use these boxes and lines..
Crow’s foot is very easy to understand and follow. Relationships and cardinality can be visualized easily in Crow’s foot notation.
Overall Crow foot’s looks more simple, visual and descriptive compare to the Chen Notation. It can be understood easily by the end users and does not take too much space and i believe it should be set as standard for ERD diagrams.
1- Chen (1976) The Entity-Relationship Model–Toward a Unified View of Data (Online) Available at: http://www.csc.lsu.edu/~chen/pdf/erd-5-pages.pdf [Accessed at: 28.9.2013]
2- Köhler (2007) Chen vs. Crow’s feet notation (Online) Avaiable at: http://ce-gmbh.com/Databases/ChenVsCrow.htm [Accessed at: 28.9.2013]
3- Picture (online) Avaible at: http://creately.com/blog/wp-content/uploads/2012/03/ER-Diagram-Elements.jpeg [Accessed at: 28.9.2013]
4- Coronel, C., Morris, S. & Rob, P. (2011), Database Systems: Design, Implementation, and Management, 9th Ed, Boston, MA: Course Technology, Cengage Learning.