Diagram Conventions   «Prev  Next»

Types of ER Diagrams

Exercise results This exercise asked you to match examples of the three best-known ER diagrams with their definitions.
Following are the correct matches:
  1. IDEF1X
  2. Crowsfoot
  3. Chen

I. IDEF1X

So they ask "What is IDEF1X ?" or "What does this symbol mean ?" and consequently we go off on a side trip.
Here is a standard answer.
This page introduces IDEF1X and identifies the notation, so that standard-compliant Data Models can be read and fully understood.
There are many closely related subjects, which require formal education; that is not provided here:
  1. the Relational Model 1
  2. Normalisation 2, 3 in general, and the specific Relational Normalization identified in the Relational Model.
  3. The IDEF1X Methodology or Relational database design

IDEF1X Method Notation

Context and Purpose

  1. IDEF1X stands for Integrated Definition, and belongs to a set of six such definitions, each of which is tightly constructed and integrated with the others.
    1 is for Information Modelling, 1X is for Relational Data Modelling.
  2. It is a stricter implementation of the Relational Model by Dr E F Codd. It is based on the theory and techniques invented by Codd (the RM) and Dr P P S Chen (the Entity Relation Model). The initial technique (methodology) was developed by R R Brown, T L Ramey and D S Coleman. It was progressed and developed to this diagrammatic form by Robert Brown in the early 1980s.
  3. It is a methodology, specifically for modelling Relational databases.
  4. It was accepted and declared as a standard by National Institute of Standards and Technology (FIPS 184) in 1993. (IDEF0 may be used for Function Modelling.)
  5. It allows the data to be modelled:
    1. in the context of the whole organisation
    2. independent of the applications that use it
    3. completely (exposing all complexities and subtleties).
    4. using a standard notation (set of symbols) and nomenclature, so that all teams and users are served. This eliminates ambiguity, and ensures that the complete set of information (identified by the standard) is communicated in the single diagram.
  6. Competent use of the Methodology produces databases that are easy to change and extend, as well as easy to use, by users and future applications. The result is a more genuinely Relational database, which allows expansion, rather than replacement due to the limitations otherwise
  7. IDEF1X is widely accepted in organisations that require compliance to Standards (Government departments, US DoD, aircraft manufacturers, large banks). It is viewed as a mark of quality, a genuine Relational database.

II. Crows Foot

Detailed Diagram outlining Crows Foot Notation

Relational Database Design

Chen Notation

Figure 7.3 shows the different types of relationships using two ER notations: the original Chen notation and the more current Crow’s Foot notation. The left side of the ER diagram shows the Chen notation, based on Peter Chen’s landmark paper. In this notation, the connectivities are written next to each entity box. Relationships are represented by a diamond connected to the related entities through a relationship line. The relationship name is written inside the diamond.
The right side of Figure 7.3 illustrates the Crow’s Foot notation. The name “Crow’s Foot” is derived from the three-pronged symbol used to represent the “many” side of the relationship. As you examine the basic Crow’s Foot ERD in Figure 7.3, note that the connectivities are represented by symbols.
For example, the "1" is represented by a short line segment, and the "M" is represented by the three-pronged "crow's foot." In this example, the relationship name is written above the relationship line.
In Figure 7.3, entities and relationships are shown in a horizontal format, but they may also be oriented vertically. The entity location and the order in which the entities are presented are immaterial; just remember to read a 1:M relationship from the "1" side to the "M" side.
The Crow's Foot notation is used as the design standard in this book. However, the Chen notation is used to illustrate some of the ER modeling concepts whenever necessary. Most data modeling tools let you select the Crow’s Foot notation.

Figure 7.3 shows the Chen model on the left and the Crow's foot on the right.