4.1 Notations

For systems analysis and design, there are several modeling notations that can be used. Two of the most predominant modeling notations are:

  • Entity-relationship notation
  • Unified Modeling Language

An entity-relationship diagram (ERD) is the classic notation used for (relational) database design. The Unified Modeling Language (UML) stems from object-oriented application development. There are more notations that exist, but this work only covers ER and UML diagrams.

Modeling basically works this way: you usually start by modeling the logical elements, associate them with each other, and declare attributes for both, elements and relationships. This is a recurring process. Both notations offer similar modeling elements, which allow you to achieve similar results. However, both notations use different terminology. Here are some crucial definitions for ER and UML diagrams:

  • Logical Elements (Subjects):

    In ER models, the basic modeling elements represent entity sets. In UML, the logical elements represent classes. Note, that the logical elements in ER and UML models only relate to each other indirectly. Entity sets and classes are not equivalent, as depicted in the 2. Terminology section. Entity sets are data containers, while classes are blueprints for ojects.

  • Relationships:

    A relationship is an umbrella term for all kinds of links between the logical elements. The classic ER term for them is the relationship, too. In UML, a relationship is called an association. The UML further defines two sub types of associations: aggregation and composition. These distinctions are missing in ERDs, but can be added by using naming conventions.

    Inheritance relationships are called hierarchical relationships in ER notation. In UML, they are differentiated into generalization and realization. Generalization is for class-class inheritance whereas realization refers to an interface-based inheritance type. ER diagrams only know about class-class inheritance, because interfaces don't have any attributes.

  • Attributes:

    Every element or relationship has a defined set of properties. In both the relational and object-oriented paradigms they are called attributes. Attributes usually consist of an identifier and a data type.

ER and UML diagrams can be used on the conceptual, logical, and physical levels. ERDs have developed several sub notations regarding relationships over the years. I will focus on the traditional ERD Chen notation in this work, but I have created a special area at the end of the next chapter on 5.6 Epilog on Relationship Notations.

Last updated: 2010-10-13