Is ERD considered a kind of UML diagram?

Question

Are Entity Relationship Diagrams(ERD\'s) considered a type of UML diagram or are they a separate thing?

Answer 1

I have a different opinion to Jarek. There is nothing stopping you modelling data using UML. Remember that UML is not new at its foundations, It represents the evolution of a number of types of diagrams which had already existed (ERDs included), and an attempt to standardise these notations to create a consistent language for the communication of system design.

ERDs are used to model (largely) a static model of a system. They show (unsurprisingly) the relevant entities in a system and articulate the manner in which they are related.

In UML Class Diagrams were born from ER diagrams to represent the relationship between classes. If you consider that (in its simplest form) a class is the combination of data and methods, by ignoring methods you are left with a data model (albeit with a syntax which varies from traditional ERDs).

So ERDs correlate to Class Diagrams in UML if you choose to exclude methods and those aspects of the model designed to indicate data protection.

However UML itself serves a broader purpose, as a family of modelling tools that collectively can be used to model both the static and dynamic nature of a system. It includes diagrams that facilitate for a more complete model of a system than you can get through describing its entities (or data):

• Class Diagrams for static object relationship modelling,
• Object Diagrams for dynamic mnodelling of object/instance relationships
• State Machine/Diagrams for modelling how a system transitions through states
• Sequence and Collaboaration diagrams to demonstrate how components of a system will work together
• Activity Diagrams (replacing flow charts) to demonstrate/document imperative flow
• and a few others I'll leave you to investigate (check out http://www.agilemodeling.com/essays/umlDiagrams.htm for an overview)