Entity relationship diagrams is type of data modelling that graphically represents objects or concepts within an information system. They are commonly used in software engineering to be implemented in databases such as a relational database, the diagram is primarily constructed from entities and the relationships between them. An entity is simply defined as an aspect of the real world or something that exists either physically or logically, one way to describe entities is to think of them as nouns.
Relationships however can be though of as verbs, something that creates a link between the two nouns, below is a simple example of how the components are used:
The relationship in the middle creates a link between the two entities to create a process.
An attribute is simply something you can add on to either a entity or a relationship to describe the component further, for example an attribute you could give to a customer entity is a phone number or an email address. An attribute you could give to a ‘signed’ relationship would be a ‘date’ as seen below:
Conceptual Data Model: The conceptual data model contains the least amount of detail but is effective in establishing a general scope of what needs to be included in the model. It is also used to form relationships between ER models to establish a foundation for model integration.
Logical Data Model: Contains more detail than the conceptual data model, commonly used in business environments as the entities and attributes are normally business names. LDM’s are also normally independent from technology such as Data Base Management Systems (DBMS).
Physical Data Model: Physical Data Models are normally developed to be instantiated as a database meaning that each PDM must be detailed enough to create a database from. PDM’s are technology dependent and can also be used to design modifications for the relational database.